The eggs of Elasmobranchs are deposited singly or in pairs at considerable intervals, and the period of egg-laying is prolonged over a considerable part of the year. In most other Fishes, as in Teleosts, the period of spawning is limited to a few months, usually in the spring and summer in temperate latitudes; and in the case of a single Fish it may last only a few days or weeks, but the number of eggs produced is often enormous. Thus, in a Ling 61 inches long and weighing 54 pounds the ovaries contained 28,361,000 eggs. A Turbot, 17 pounds in weight, had 9,161,000 eggs; and a Cod of 21½ pounds 6,652,000. The least prolific of the British food Fishes is the Herring, in which the number of ovarian eggs varied from 21,000 to 47,000 in four specimens examined.^[490] The extraordinary fecundity of many Fishes seems to bear no relation to the relative abundance of the Fishes themselves, but rather it is to be associated with certain disadvantages attendant on the sexual relations of Fishes, involving a considerable waste of the sex-cells, while in many Fishes it no doubt helps to compensate for any subsequent mortality among the larvae, which may result from an uncertain and precarious food supply and from the attacks of enemies. Whenever internal fertilisation is the rule, or when, as in nest-building and marsupial Fishes, the propinquity of the sexes in the breeding season ensures the fertilisation of a larger proportion of the eggs and the protection of the young, the number of eggs produced is small.

The male sex-cells or spermatozoa are essentially similar to those of other Vertebrates, although in different Fishes they may vary in such details as length, and the shape and size of the head, which may be rod-like and wavy, elliptical or globular.

As a rule, in Fishes females are more numerous than males, and generally they are larger, but to both statements there are notable exceptions. The relations of the sexes in the breeding season are usually very promiscuous, especially in those Teleosts which discharge their sex-cells while swimming together in shoals. A female may, however, consort with several males (polyandry), or a male with several females (polygamy); or, as in some of the nest-building Fishes (e.g. Gastrosteus), there are not wanting examples of the pairing of one male with one female (monogamy).

Fishes often migrate at the commencement of the breeding-season to localities most suitable for the deposition of the eggs. Many marine species seek banks or shallower water near the shore, and some, like the Salmon and the Sturgeon, are anadromous, and ascend rivers for long distances to deposit their spawn.

In all Fishes except the Elasmobranchs and a few Teleosts the fertilisation of the eggs takes place in the water after their extrusion, the male depositing its seminal fluid over the eggs or in their neighbourhood. The waste of the sex-cells is often, no doubt, very considerable, especially when the eggs are adhesive and fixed, and the seminal fluid is liable to drift at the mercy of tides and currents. With pelagic ova the waste is perhaps not so great, inasmuch as the eggs as well as the spermatozoa would probably drift at the same rate and in the same direction. Liability to waste must also be greatly diminished in many Fishes by their habit of living in shoals, or of congregating together in the breeding season, in which they are sometimes aided by their power of emitting characteristic sounds, and in the case of nest-building Fishes by the still more intimate relations of the sexes. Even when the liability to waste is very great, compensation may be afforded by exceptional fecundity. The copulation of the sexes and the internal fertilisation of the eggs occur only in Elasmobranchs and some Teleosts. The copulatory organs of Elasmobranchs are the so-called "claspers" with which the males are provided. Some form of copulation is probably the rule in the viviparous Teleosts, where the eggs are fertilised in the oviducts, or even while they are still in the ovaries, and the young are born alive. As mentioned above, an intromittent organ is often formed by the prolongation of the genital or the urinogenital orifice into a papilla, or a longer or shorter tube.^[491] Some Cyprinodontidae^[492] (e.g. Anableps) have the anterior part of the anal fin modified in the male to form an intromittent organ along which the urinogenital canal runs (Fig. 374). In the females the genital aperture is covered by a special scale, which is free on one side and not on the other. "The male organ in some individuals is turned to the right, in others to the left, and in some females the opening beneath the special scale is to the right, in others to the left. Copulation thus takes place sideways, a left-sided male pairing with a right-sided female, and vice versa."^[493] The anal fin also forms an intromittent organ in the "Half-beak" (Hemirhamphus). In a genus (Girardinus) of the same family the anal fin is modified to form an apparatus for holding the female during sexual congress.^[494] The singular method of fertilisation practised by the males and females of Callichthys paleatus is referred to elsewhere.^[495]

With the exception of the pelagic Antennarius, which builds its nest in the Sargasso weed in mid-ocean, nest-building and parental solicitude for the young are confined to freshwater Fishes and to marine forms with demersal ova. Pelagic ova must necessarily be beyond the scope of parental care. As a rule it is the male which acts as guardian nurse, the female troubling herself but little about the fate of her eggs or her offspring.

Perhaps the more primitive form of parental foresight is exhibited by those Fishes which, like the females of the Salmonidae, make a furrow in the gravelly bottom of a running stream for the reception of the eggs, and then cover them over with a layer of gravel, or like the Siluroid Arius australis, of the Burnett river in Queensland, which deposits its eggs in circular excavations in the sandy bed of the river and covers them with layers of large stones. But in neither case does it appear that either the male or the female takes any further interest in the eggs or in the young when hatched. Without actual nest-building, or even the preparation of a place for their reception, the eggs may be protected in various ways by the male. The common British Gunnel or Butter-Fish (Pholis gunnellus) rolls its eggs into a rounded mass by coiling its body round them, the male and female taking possession of them alternately. The little clumps of eggs are then deposited in holes made by the boring Mollusc, Pholas. Some British Blennies attach their eggs in a single layer to the sides of cavities in rocks, or between stones, where they are watched over by the male parent. The eggs of the Lump-Sucker (Cyclopterus lumpus) are attached in masses to rocks or to piles and guarded by the male, who aerates them by keeping up a flow of water over the spawn through the action of his pectoral fins. When hatched, the young fry cling to the body of their watchful parent by their suckers. A more decided approach to nest-building is exhibited by the Sand Goby (Gobius minutus). In this species the male scoops out the sand from beneath an empty shell, generally that of a Pecten, and the female deposits her adhesive eggs on the under surface of the shell.

The male remains on guard, and by the movements of its pectoral fins promotes the aeration of this rude form of nest. References to some of the more striking examples of true nest-building in Fishes will be found in the systematic part of this volume, especially in those chapters treating of the Dipnoi and Amiidae, and such Teleosts as the Mormyridae, Osteoglossidae, Siluridae, Gastrosteidae, Centrarchidae, Osphromenidae, Labridae, and Antennariidae. Other illustrations of parental care are to be found in the development of marsupial pouches or grooves for the reception of the eggs in the males of the Syngnathidae (Fig. 387) and the females of the Solenostomidae, and the use of the oral cavity for a similar purpose in the males, rarely in the females, of some Siluridae, and the males or females, according to the species, of the Cichlidae. The singular method by which the female Aspredo safeguards both her eggs and her progeny is referred to on p. 596. The Cyprinoid, Rhodeus amarus (the "Bitterling" of Central Europe), is unique in the means which it adopts to secure the same result.^[496] By means of its long ovipositor the female Fish deposits its eggs in the mantle cavity of a Unio, or of an Anodon. Here they are fertilised by spermatozoa carried in through the inhalent siphon of the Mollusc with the inspiratory water current, and they complete their development in the gill-cavities (Fig. 237).^[497]

The time which elapses between the fertilisation of the egg and the hatching out of the young Fish varies greatly in different Teleosts. The eggs of some Clupeidae hatch in a very short time, two to three days in the Anchovy, and three to four days in the Sprat. In most of the British marine food Fishes the period rarely exceeds twelve to fourteen days. The larger demersal eggs with much food-yolk are longer in hatching; in the Salmon the time ranging from thirty-five to one hundred and forty-eight days. A low temperature lengthens the time. The eggs of the Herring which hatched in eight to nine days at a temperature of 52° to 58° F. took forty-seven days in water at 32° F.

The extent to which the development of the embryo proceeds while it is still enclosed in the egg-membranes, and consequently the condition of the embryo when hatched, depends largely but not exclusively on the quantity of food-yolk which is present in the egg and available for the nutrition of the embryo during its earlier stages. Embryos hatched from pelagic ova are very small and imperfectly developed. The mouth is usually not yet formed. The median fins, which later become isolated, are continuous, and the caudal fin is diphycercal, although it subsequently becomes homocercal after passing through a heterocercal stage. The blood is colourless, and even the gill-clefts may at first be lacking. In this condition the newly-hatched Fish is nourished at the expense of the residual food-yolk, which is enclosed in a yolk-sac projecting from the ventral surface of the body (Fig. 238). As the yolk is gradually used up the mouth is formed, and the young Fish feeds on the minute organisms of various kinds living in the sea, and by degrees the form, proportions, and structure of the more mature Fish are acquired. In the case of the larger demersal eggs the young are not only longer in hatching, but when hatched they are larger and more advanced in development. The young of many Fishes are provided with larval or provisional organs, and they may be so unlike the adult in other respects that their subsequent development assumes the form of a more or less striking metamorphosis. As examples of larval organs, mention may be made of the adhesive or cement organs of the larval Chondrostei and Holostei, and of the Dipnoi (e.g. Protopterus and Lepidosiren), and also of a Teleost, probably the Mormyrid (Hyperopisus bebe, Lacép);^[498] the cutaneous gills of the Crossopterygii and some Dipnoi; the so-called external gills of such Teleosts as Cobitis, Gymnarchus (Fig. 239), and Heterotis, which are singularly like those of Elasmobranchs; and the defensive spines which are developed on the scales or scutes of the trunk, and the dermal bones of the skull, in the young of some Plectognathi. The most striking metamorphosis to be found in Fishes occurs in the Flat-Fishes and in the Eels, an account of which will be found in other parts of this volume (pp. 685, 602).

The only examples of viviparous Fishes occur in certain families of Elasmobranchs,^[499] and in five families of Teleosts, viz. the Blenniidae, the Cyprinodontidae, the Scorpaenidae, the Comephoridae, and the Embiotocidae.^[500] In the Teleosts mentioned the eggs are fertilised while they are still either in the ovarian ovisacs or in the cavity of the ovary, and their development may take place in either position. In such Cyprinodonts as Gambusia and Anableps the embryos are developed in the ovisacs, but as a rule both fertilisation and development occur in the ovarian cavity. During a prolonged gestation the young are nourished partly by the food-yolk present in the eggs, and partly by a nutritive secretion derived from the ovarian walls or from the epithelial wall of the ovisacs as the case may be. In Anableps the secretion of the walls of the ovisacs is absorbed by papillae developed on the surface of the yolk-sac of the embryo along the course of its blood-vessels. The eggs of the Embiotocidae have little food-yolk, and the embryos are mainly nourished by the secretion of the ovarian walls, which is swallowed by the embryo and absorbed by villi on the inner surface of the intestine. The number of young produced varies considerably. In the Embiotocidae the ovarian cavity contains 40 to 50 young. The viviparous Scorpaenid, Sebastes norvegicus of Northern Europe, produces, it has been estimated, about 1000 young, while the Blenny (Zoarces viviparus), the only other European viviparous Teleost, produces from 20 to 300 or more, according to the size of the female. In the Blenny the eggs are hatched in about twenty days, but the young are not born until about four months after fertilisation, when they are about an inch and a half long, and in every outward respect similar to the adult Fish.

Besides the distinction between the sexes resulting from the different nature of their gonads and sex-cells, the males and females are often distinguished by secondary sexual characters ("sexual dimorphism"^[501]). As mentioned above, females are usually larger as well as more numerous than the males, although in one or both respects the reverse may be the case. Secondary sexual characters are best marked in Teleosts, where they are generally related to the special rôle which each sex takes in the deposition and fertilisation of the eggs, and the nurture and protection of the young, of which examples have already been given. To a more limited extent they may be associated with the struggle of the males for the females, and in at least a few Teleosts the exuberant coloration of the males in the breeding season suggests that instances of courtship and sexual selection are not altogether wanting.^[502]

Although the vast majority of Fishes are dioecious, instances of functional hermaphroditism are not unknown in a few Teleosts.^[503] Species of the Percoid genus Serranus (e.g. S. cabrilla, S. hepatus, and S. scriba) are invariably hermaphrodite and self-fertilising. Chrysophrys auratus is an example of successive hermaphroditism, the male and female sex-cells ripening alternately. As an occasional variation hermaphroditism has been recorded in several other Teleosts, including amongst others such well-known Fishes as the Cod, the Mackerel, and the Herring. The relations of the gonads in hermaphrodites is subject to much variation. In the Cod, for example, the testes may be double, each being continuous with the hinder end of the ovary of its side, or there may be only a single testis confluent with the anterior or the posterior portion, or with some other part of the surface, of either the right or left ovary. In other Teleosts individuals occasionally present themselves with a testis and an ovary on opposite sides.

CHAPTER XVI

CYCLOSTOMATA (SYSTEMATIC)

CLASS I. CYCLOSTOMATA

The Cyclostomata, or, as they are sometimes called, the Marsipobranchii, from the pouch-like, or rather sac-like, shape of their branchial clefts, are divided into two orders, the first comprising the "Hag-Fishes" or "Borers," and the second the Lampreys.

Order I. Myxinoides.

The Hag-Fishes are probably the most primitive of all existing Craniates. The mouth is nearly terminal, and there is no buccal funnel. The naso-pituitary involution communicates behind with the oral cavity and functions as a channel for the in-streaming water-current to the gills. Four pairs of short tentacles, supported by a special tentacular skeleton, are present in relation with the mouth and the terminally-placed naso-pituitary orifice. The gill-sacs open directly into the pharynx. The branchial basket is but feebly developed, and at the most it is only represented by small isolated cartilages in relation with the external branchial apertures. The lingual apparatus is remarkably developed. Besides the lingual teeth there is only a single dorsal tooth in the roof of the mouth. The dorsal arcualia are restricted to the tail, or they extend for a short distance only into the trunk. A spiral valve is absent. There is a row of mucus-secreting sacs along each side of the body. The brain has no obvious cerebral hemispheres, nor a cerebellum. Only one semicircular canal is present in the auditory organ. The eyes are degenerate, and the usual eye-muscles with the cranial nerves supplying them have atrophied. The embryonic pronephros is retained in the adult. The eggs are large; segmentation is meroblastic; and development is direct, without a larval metamorphosis. Two families can be distinguished.

Fam. 1. Myxinidae.—Gill-sacs not exceeding six pairs, with a common external aperture on each side of the body.

The family includes a single genus, Myxine, of which the common Hag (M. glutinosa) from the North Atlantic is the best known species (Figs. 92, A, and 240). This Hag-Fish occurs off the coasts of Northern Europe, including the British Isles, as well as on the Atlantic sea-board of North America,^[504] southwards to Cape Cod. Other species are found off the coasts of Chili and Japan. Myxine is quasi-parasitic in its habits, boring its way into the bodies of large Fishes. By means of its rasping "tongue" it devours all the soft parts of its prey, leaving little more than a mere shell of skin and bones. The Fishes usually attacked are the Cod and other Gadoids, but the Sturgeon is not immune, and the presence of a Hag in the abdominal cavity of a Shark (Lamna cornubica) has been recorded. Myxine has the reputation of being very destructive to Fishes caught on lines, and it is said that whole "catches" have been destroyed by its depredations, so that North Sea fishermen have been forced to change their fishing-ground. To what extent the Hags attack Fishes which are living and free is somewhat uncertain, but the little evidence obtainable seems to point to the conclusion that, as a rule, they only prey on Fishes when the latter are hooked or netted, or injured or dead. When not seeking food the Hag lives in the mud of the sea-bottom at depths ranging to nearly 350 fathoms. They are able to swim very rapidly in an undulatory eel-like fashion. M. glutinosa may grow to a length of nearly two feet. The Hag has been described as a protandrous hermaphrodite, that is, it is first a male and then a female, the gonad of the young first producing spermatozoa, and at a later period becoming an ovary and giving rise to eggs. This view has hitherto met with general acceptance, but it has recently been urged with some force that the presence of the two kinds of sex-cells in a young animal is no proof of functional hermaphroditism, since it is not uncommon "to find immature eggs in the testis of many Vertebrates (Teleosts, Petromyzon, Amphibia), where the assumption of hermaphroditism, to say nothing of its protandric form, is entirely unwarranted."^[505] Myxine produces eggs similar to those of Bdellostoma. Nothing is known of its breeding habits, or of its embryology.

Fam. 2. Bdellostomatidae.—Gill-sacs 6-14 pairs, all with separate external orifices. Bdellostoma (Fig. 92, B) is found on the Pacific sea-board of both North and South America, at the Cape of Good Hope, and on the coasts of New Zealand. The numerical variation of the gill-sacs in different species, and in different individuals of the same species, and even on opposite sides of the same individual, is very remarkable. Out of 354 examples of the Californian species (B. stouti) examined by Dr. Ayres,^[506] 101 had 11 gill-sacs on each side; 26 had 11 on one side and 12 on the other; 208 had 12 on each side; 11 had 12 on one side and 13 on the other; and 8 had 13 on each side. Occasional specimens may have 14 gill-sacs on each side. The variations are apparently quite independent of size, age, or sex; and when the gill-sacs are asymmetrically developed, the additional sac may be either on the right side or on the left. In the Chilian species there are 10 gill-sacs on each side, but in the species from the Cape of Good Hope the number is reduced to 6 or 7. Bdellostoma closely resembles Myxine in its habits and mode of feeding. The Californian species attaches itself to the gills or to the isthmus of large Fishes, and then rapidly bores its way into the body, devouring the viscera and muscles but leaving the skin intact. It usually attacks large Flounders and species of Sebastodes, and it is especially destructive to Fishes taken in gill-nets. At Monterey every net in the summer contains the empty shells of eviscerated Fishes, and when these are taken out of the water the Hag scrambles out with great alacrity. Large fishes of even 30 pounds weight are often captured without either flesh or viscera, and it cannot be supposed that they entered the net in this condition.^[507] The species lives on the sea-bottom most abundantly at a depth of 10-20 fathoms, but becomes rarer as the water deepens or becomes shallower.

The eggs of the Californian Bdellostoma are large, varying in size from 14.3-29 mm. in length, and from 6.8-10.5 mm. in width, and each egg is enclosed in a horny egg-case secreted by the epithelium of its ovarian ovisac^[508] (Fig. 241). At each pole of the egg-case there is a tuft of numerous horny filaments which end in 2- 3- or 4-hooked, anchor-like extremities. In the centre of the tuft of filaments at the animal pole of the egg the egg-case is perforated by a micropyle, and a little below this point the case is encircled by an opercular groove, which enables the polar portion to be thrown off like a cap at the time of hatching, so as to allow the young Bdellostoma to make its escape. The large size of the egg, which almost completely fills the cavity of the egg-case, is due to the fact that it consists mainly of food yolk, the germinal protoplasm containing the nucleus forming only a small hillock near the inner extremity of the micropyle. Bdellostoma spawns during the greater part of the year, but chiefly in the early summer, and probably about 20 eggs are deposited at one time, generally on a shelly or rocky bottom. After deposition the eggs become connected together in long chains or clusters by the interlocking of their polar hooks. Fertilisation takes place after extrusion, and the segmentation is meroblastic and discoidal, much as in Teleosts. The embryo completes its development within the egg, and when hatched it is a miniature of the adult.

Order II. Petromyzontes.

In the Lampreys there is a large suctorial buccal funnel leading behind and above into the mouth, which is supported by special cartilages, and furnished with a marginal fringe of small cirri. Numerous horny teeth are present on the inner surface of the funnel as well as on the tongue. The naso-pituitary involution forms a caecum and does not communicate with the mouth. The gill-sacs, seven in number, open externally by separate orifices, but internally they open into a median branchial canal, situated below the oesophagus and opening into the mouth in front. There is a well-developed branchial basket. Dorsal arcualia are present throughout the precaudal as well as in the caudal region. A rudimentary spiral valve is present. The brain consists of parts usually present in other Craniates, including cerebral hemispheres and a cerebellum. The auditory organ has two semicircular canals, and the eyes are not degenerate. The pronephros is suppressed in the adult. The eggs are small; the segmentation is holoblastic; and there is a larval metamorphosis. There is but one family.

Fam. 1. Petromyzontidae.—The family has a nearly world-wide distribution. Most Lampreys are marine, although to a greater extent in some species than in others, but all of them seem to ascend rivers for spawning. The genus Petromyzon is characteristic of the northern hemisphere, where it is represented by various species on the coasts and in the rivers of Europe, West Africa, Japan, and North America. Three species, widely distributed in Europe, occur in the British Isles, viz.:—the Sea-Lamprey (Petromyzon marinus), which may reach or even exceed three feet in length, and is also found on the west coast of Africa and on the Atlantic coast of North America; the "Lampern" or fresh-water Lamprey (P. fluviatilis), about 18 inches long; and the Sand-Pride, Sand-Piper, or lesser freshwater Lamprey (P. planeri), usually less than a foot in length. Ichthyomyzon, Bathymyzon, Entersphenus, and Lampetra are also northern forms, collectively distributed along the Atlantic and Pacific coasts and in the rivers and great lakes of North America.^[509] Other Lampreys occur only in the southern hemisphere. Geotria is common in the rivers of Chili, Australia, and New Zealand; and another genus, Mordacia, has a parallel distribution, being found on the coasts of Chili and Tasmania. A new genus and species from Chili has been recently described under the name of Macrophthalmia chilensis.^[510] This Lamprey, which is only 107 mm. in length, has remarkably large eyes (2.5 mm. in diameter), vertically compressed gill-clefts, and a simple dentition resembling that of Myxine. All Lampreys are carnivorous. They feed by attaching themselves to the bodies of Fishes by their suctoral buccal funnels, and then rasping off the flesh with their lingual teeth. While thus engaged they are carried about by their victims. Salmon have been captured in the Rhone with the marine Lamprey attached to them. The Lamprey usually keeps near the bottom, either swimming with a graceful serpentine movement, or attached to stones by the buccal funnel.

In the spring the Sea-Lamprey ascends the rivers to spawn, and, after depositing its eggs in furrows which it excavates in the river-bottom, it returns to the sea. The river-Lampreys spawn in the smaller streams and brooks. The North American Brook-Lamprey, Petromyzon (Lampetra) wilderi, which is found in the neighbourhood of New York, deposits its eggs on the gravelly bottom of a brook, in a small gravel-filled hole lying between a number of large rounded stones^[511] (Fig. 243). In the vicinity of the "nest" some ten to twelve Lampreys congregate, the males, however, being much more numerous (five to one) than the females.

Much energy is spent by both sexes in moving stones by lifting them with the buccal funnel, but it is not always clear that this is done to circumscribe the nest, or to remove impeding obstacles. Eventually, a male attaches himself to the back of the head of a female, who at the same time is holding fast to a stone. The male then rotates its body so that the urino-genital papilla is brought near the genital orifice of the female, and the simultaneous extrusion of eggs and spermatozoa at once follows. Owing to the small amount of food-yolk which they contain the eggs of the Lamprey (e.g. P. planeri) are small, measuring about 1.1-1.2 mm. in length, and from 0.9-1.0 mm. in width. There is a micropyle at the animal pole of the egg, but the characteristic horny egg-case and the polar hooks of the Myxinoids are both wanting. The embryo hatches out as a larva known as the "Ammocoetes." At this stage of its development the larva lacks several of the most striking features which characterise the adult, and it is highly probable that the Ammocoetes represents a stage in the evolution of Vertebrates in some respects intermediate between Amphioxus and a very primitive Craniate. The mouth of Ammocoetes is bounded laterally and in front by a curious hood-like upper lip, and behind by a short transverse lower lip (Fig. 244). The eyes are deeply seated and rudimentary, and as visual organs they are useless, but the parietal eye is well developed. As in the adult, there are seven pairs of gill-sacs, but they open internally into a pharynx, directly continuous behind with the rest of the alimentary canal, and there is no dorsal oesophagus. Like the skull, the branchial basket is still very rudimentary. The dorsal and caudal fins are continuous. A gall-bladder is present, and also a bile duct opening into the gut. In its mode of life, and especially in the manner in which it obtains its food, Ammocoetes presents a most remarkable resemblance to Amphioxus and the Ascidians. In the median line of the pharyngeal floor there is an open groove, the hypopharyngeal groove or endostyle, and a tract of ciliated cells along the dorsal wall represents a hyperpharyngeal groove: connecting the two in front there is a peripharyngeal ciliated groove.^[512] The Ammocoetes feeds on small food particles carried through the mouth into the pharynx by currents of water produced by ciliary action. The food becomes entangled in strings of mucus probably secreted by the cells lining the endostylar groove. The mucus is then swept upwards in the pharyngeal groove, and finally wafted backwards to the stomach and intestine by the cilia of the hyperpharyngeal band. The skin exhibits the remarkable peculiarity of containing a peptic ferment capable of digesting proteids in a .2 per cent solution of hydrochloric acid. As the larva lives buried in the mud, the epidermic secretion probably helps to keep the skin free from bacteria, microscopic spores, and fungoid, or other parasitic growths.^[513] The young Lamprey lives as an Ammocoetes from 3-4 years, and then in the course of a few weeks in the winter it undergoes a metamorphosis, losing its larval characters and acquiring the structure and habits of the adult. During this period the buccal funnel is completed and teeth are developed. The eyes approach the surface and become functional. The continuity of the median fins becomes interrupted. The endostylar groove becomes transformed into a thyroid gland, the gall-bladder disappears, and the bile duct becomes obliterated and changed into a mass of small follicles. The skull and branchial basket complete their development. At the same time the pharynx loses its connection with the rest of the alimentary canal and remains as the branchial canal. The so-called oesophagus of the adult is apparently a new formation which grows forwards and acquires a connection with the mouth. It is probable that it represents a hyperpharyngeal groove constricted off from the dorsal wall of the pharynx.

Both the marine Lamprey and the "Lampern" are captured for food, either by nets or wicker traps. Formerly the Lampern was taken in enormous numbers in several British and Irish rivers, especially in the Severn from February to May, and in the Thames during May and June, but for various reasons the supply has much diminished in recent years. The Lampern makes excellent bait for Cod and Turbot, and for this purpose large numbers used to be taken in the Trent and Thames for despatch to Grimsby and other fishing ports.^[514]

CHAPTER XVII

CLASS II. PISCES.

Sub-Class I. Elasmobranchii.

In both the ancient and the modern Sharks, Dog-Fishes, and Rays, the exoskeleton takes the form of a more or less uniform investment of dermal denticles or "shagreen." The endoskeleton is wholly cartilaginous or partially calcified, and there are neither cartilage- nor membrane-bones. The vertebral column is acentrous or chordacentrous, generally with alternating basi- and inter-dorsal elements, and terminating in a heterocercal tail. The skull is usually hyostylic, very rarely amphistylic or autostylic, and the lateral halves of the primary upper jaw (palato-quadrate cartilages) usually meet in a highly characteristic median symphysis beneath the base of the skull. Branchial arches and clefts are five to seven in number, and the clefts are separated by complete interbranchial septa, which, as a rule, are continuous externally with the skin. An operculum is developed only in the Holocephali. A pelvic girdle is present. With rare exceptions the pectoral fin is uniserial. The pelvic fin is invariably uniserial. The exoskeletal supports of all the fins consist of ceratotrichia, and, when present, the fin-spines are invested by enamel. Claspers are generally present in the males.

In the surviving members of the group the nostrils retain their primitive ventral position. There is a conus arteriosus with several rows of valves. A spiracle, often furnished with a spiracular pseudobranch, is generally present, and, as a rule, there is a hyoidean hemibranch supplied with venous blood from the ventral aorta. The gill-filaments are attached throughout their length to the interbranchial septa. There is an optic chiasma. An air-bladder is not developed. The intestine has a spiral valve, and there is a cloaca. The gonoducts in both sexes are derived from the kidney system. The ova are large, few in number, and enclosed in horny egg-cases, and they are fertilised before extrusion. The segmentation is meroblastic, and the embryo is furnished with long external gills.

The Elasmobranchs are for the most part active predaceous Fishes, living at different depths in the sea, from the surface to nearly a thousand fathoms, and ranging from mid-ocean to the shallower waters round the coasts in almost every part of the world. Although typically marine, they sometimes ascend rivers beyond the reach of tides, and a few are permanent inhabitants of fresh water. They are most abundant in tropical and subtropical areas, where they also attain their greatest size, and are numerous in temperate regions, but there are some species which are typically Arctic. None of them are small, and some of the Sharks are the largest of living Fishes. All are carnivorous, but so diversified is their food that in different species it may range from other Fishes of no mean size to Molluscs, Crustaceans and other Invertebrates, or even to plankton. In their breeding habits the Sharks and Dog-Fishes present many interesting features. Unlike the generality of Fishes, the eggs are fertilised internally as a sequel to the copulation of the sexes. For this purpose the males are furnished with special intromittent organs, the myxopterygia or so-called claspers, which are developed as modifications of the hinder portions of the pelvic fins.^[515] Each clasper is supported by an internal skeleton, consisting of several cartilages derived from the radialia of the fins, and is traversed along its inner aspect by a groove. When sexual congress takes place the claspers are thrust through the cloaca of the female into the oviducal orifices, and in some instances it is probable that they are retained in this position by hook-like denticles developed at their free extremities. The seminal fluid then flows along these conduits into the oviducts, in the upper portions of which it meets and impregnates the eggs. After fertilisation the egg is enclosed in a dark brown horny egg-case, secreted by the oviducal gland.

As a rule each egg-case has but a single egg, but in Rhinobatus and Trygonorhina (Batoidei), both of which are viviparous, each case contains three to four eggs. Generally the egg-cases are somewhat quadrangular in shape, with the four angles, two at each end, prolonged either into short horns, or into long tapering tendrils (Fig. 246). The oval egg-cases of the Heterodontidae are remarkable not only for their size, but also for the presence of a broad spiral lamina winding round the exterior of the case from one pole to the other (Fig. 245). The majority of the Sharks, Dog-Fishes, and Rays are viviparous, that is, the young are born alive; the rest, like the Scylliidae (e.g. the common British Dog-Fishes, Scyllium canicula and S. catulus), the Heterodontidae, and the Raiidae are oviparous, that is, the young are hatched out after the extrusion of the eggs. In the oviparous species the eggs are extruded either singly or in pairs, and generally deposited on the sea-bottom. When, however, the egg-cases are provided with tendrils, as, for example, in the two British Dog-Fishes just mentioned, these organs act as anchoring filaments. When extruding an egg, the female swims round and round some piece of upright seaweed, and the curling tendrils become entwined round it in such a way that the egg becomes securely attached thereto (Fig. 246).^[516] The embryos are long in developing, and in Scyllium it may be several months after fertilisation (200 to 275 days) before they are hatched, the young Fish finally escaping through a rupture in the egg-case.