The hydroids have been called the nurses of jellyfishes. From casual observation these two forms would not be associated together, for the shrub-like organisms, which so much resemble plants that they are often collected and preserved as seaweeds, suggest only vegetable life. Examined with a glass, however, they disclose their animal nature. Along the stems, arranged in various ways, are small cups, from which protrude the numerous moving tentacles of the little polyps living within them.
Hydroids are colonies of associated animals living a communal life. The multitude of individuals composing the colony are invested with a horny covering, the perisarc, which in some genera assumes a tree-like form. Through these stems and branches runs a fleshy tube, a thread of animal substance, which connects in one living whole the zoöids, or individuals of the community. There is division of labor, as in other communities: some of the zoöids obtain the food for the colony, and have tentacles around their open mouths; others have no mouths, but reproduce the species, and at certain stages of development liberate swimming-bells, or small jellyfishes (medusæ).
The typical hydroid colony is attached by a kind of creeping stem from which arises a vertical axis, which gives off short lateral, alternate branches bearing zoöids at their ends. There is often more complex branching. The zoöids in certain genera (tubularians) are uncovered; in others (sertularians) they are incased in a glassy, cup-like, horny sheath.
Three kinds of zoöids, polyps, or hydranths—as they are indiscriminately called—are attached to the stem. Those having an [pg120] open end and a crown of tentacles are the nutritive individuals. Small, club-like dilations are immature zoöids. The blastostyles, or reproductive zoöids, are long, cylindrical, mouthless, and covered. At maturity the cover is ruptured, and the medusæ have the appearance of a pile of thin saucers attached by the middle of the convex side. When at length these saucers are set free as little medusæ, or jellyfishes, the convex side of each saucer, or swimming-bell, is called the ex-umbrella; the concave, under side, the subumbrella. From the center of the subumbrella projects the manubrium, or stomach of the animal. At the free end of the manubrium is a four-cornered mouth. From the attached end of the manubrium four tubes or canals diverge, and, extending through the animal, open into a circular canal which runs around the margin of the umbrella. When the medusa is as above described, it has reached the highest point in its development.
When the medusa has matured, it lays eggs, known as planulæ. These are spherical bodies covered with cilia (hairs), by means of which they swim about for a time; but they finally attach themselves to some object, there to grow and develop into hydroid colonies. The cycle of life is thus completed. This process is known as alternation of generation, or metagenesis, one life-history containing two quite different forms of being. The term of life of an individual is one year, the zoöphyte stage beginning in the autumn and the medusa stage in the spring.
Some medusæ, besides reproducing by means of eggs, multiply by budding, small medusæ growing on the manubrium or on the margin of the umbrella. Sarsia and Lizzia sometimes increase by budding.
The Hydrozoa are not all of the above type. In the sertularians the zoöids perish on the stem and have no medusa life, their reproductive element giving rise to the hydroid form without metamorphosis. The Trachylinæ have no hydroid life, being always free-swimming medusæ; others, the Siphonophora, live a hydroid life which is unattached, the colony floating on the ocean; the millepores secrete calcareous skeletons and always remain fixed, reproducing by budding. [pg121]
Hydroids are very abundant, but are comprised in the few groups mentioned: namely, those which live only in the fixed colonial state; those which have alternation of generation, being first hydroids and then swimming-bells, or medusæ; those which live always in the medusa state, the eggs of the jellyfish developing at once into other medusæ; and the Siphonophora, or those which have a floating colonial state, the hydroid never being attached, but floating at large and capable of locomotion, some of the colony having the function of propulsion.
Hydroids are particularly interesting as exemplifying the close resemblance that may exist in outward appearance between animal and vegetable life and as illustrations of communal life and of the alternation of generation. A few examples of different types are given below.
The members of this order agree in all essential particulars being branched colonies having two principal forms of zoöids, the nutritive and the reproductive. Some genera attain the length of several inches, or even feet; others are very small tufts growing on shells and seaweeds. The cup may completely inclose the zoöid and be close to the stem (sessile), as in sertularians; it may be on the end of a short stalk, as in campanularians; or it may not reach above the base of the zoöid, as in tubularians. The genera are based upon these differences in the perisarc.
The hydroids, like all other classes, exist in such great variety that it would be impracticable to describe here the many named species; but to recognize the genera is simple. A long tubular pedicel without a cup is characteristic of the tubularians; the campanularians have an arborescent form and bell-shaped cups on stalks; the sertularians have sessile cups; and the plumularians have a feather-like form, with zoöid-cups on one side only of the branches.
The beautiful and varied structure of these "animal plants" is most interesting, and to be fully appreciated they should be seen [pg122] in life and examined with a glass. Some species are confined to deep water, but many are littoral and to be found in tide-pools, in the chinks and crannies of rocks, under stones, and under the hanging Fucus. The horny skeletons of large varieties are frequently washed ashore, and in their tangled masses smaller living species often may be found.
This division is characterized by zoöids borne on long, slender stems which are sometimes simple and small, sometimes branching and eight to ten inches long. The zoöid has two rows of tentacles, the central one being sometimes on a kind of proboscis. The reproductive zoöids are in bunches, sometimes below the outer row of tentacles, sometimes between the two rows. The perisarc does not cover the zoöid. In color they are commonly red or yellow.
Genus Clava
C. leptostyla. This species is found growing on Fucus, on the under side of stones at low-water mark, and in tide-pools, where it often covers several feet of the surface of the rock with a delicate velvet-like carpet. It is red in color and is, apparently, a soft and tender species, but it thrives on the most exposed beaches. The colonies are cylindrical tubes about one quarter of an inch in height, rising from a creeping stem (hydrorhiza). Each tube is surmounted by a zoöid with fifteen to thirty tentacles, which is constantly changing form by its contractions. Below the tentacles are reproductive buds arranged in clusters. Common from Long Island Sound northward. (Plate XLI.)
| PLATE XLI. | |
|---|---|
| Clava leptostyla. | Coryne mirabilis. |
| Tubularia indivisa. | Eudendrium ramosum. |
| Hybocodon prolifer. | Tubularia Couthouyi. |
Genus Hydractinia
H. polyclina. The soft, pinkish covering often seen on shells inhabited by hermit-crabs. This association of two different kinds of animals is known as commensalism, and is a partnership formed for the benefit of one or both the individuals. In this case the mossy appearance of the hydroid conceals the shell, while the stinging-cells with which it is invested are weapons of defense against the enemies of the crab and also help to paralyze its prey. In return for these favors the colony is moved about, thereby obtaining perhaps better oxygenation. Originally it was thought that Hydractinia lived only on the shells occupied by hermit-crabs, and that the nomadic life was essential to its existence; but this is not the case, for it is also found growing on rocks in tide-pools. These colonies arise from a creeping stem, which forms a horny, root-like [pg123] network over a surface and develops at intervals projecting points on which the zoöids live. Each colony consists of feeding members, of reproductive members, and of a third kind which seems to have a protective function. These last are more slender than the others, and are without tentacles, but are armed with lasso- or stinging-cells. The colonies are of different sexes, the male being lighter in color than the female colonies. The eggs develop into planulæ, which swim about for a while and then give rise to other colonies. It is found from New Jersey northward, and is very abundant in Long Island Sound.
Genus Coryne
C. mirabilis. A hydroid about one inch high, growing in patches and appearing like tufts of moss on rocks between tide-marks. When highly magnified it shows club-shaped tubes with pedicels, terminating in zoöids, scattered over the swollen ends. The medusa-bud is larger than the others and is lower on the tube. It liberates a swimming-bell, which is called Sarsia. (Plate XLI.)
Sarsia
S. mirabilis. This medusa of Coryne is from one quarter to three quarters of an inch in diameter when full-grown. Its umbrella is nearly hemispherical, and from the center hangs a manubrium. From the margin of the umbrella hang four very long tentacles. The shape of its body and the length of its tentacles and proboscis are constantly changing as it moves in the water. These little medusæ are very plentiful in the spring and summer, and swim rapidly in all directions near the surface of the water.
Genus Tubularia
T. Couthouyi. This species is found in the same places as Parypha crocea. The stem is three to six inches long, and is inclosed in a horny sheath, which is more or less ringed or jointed, or it may be smooth throughout. The head, when the tentacles are expanded, measures one and a half inches in diameter. It has a proboscis covered with tentacles, disposed in series, which grow successively shorter, the last being merely papillæ. The medusa-buds hang in clusters between the outer tentacles and the proboscis. The animal grows in bunches of five to ten tubes, which spring from a creeping, tangled stem. (Plate XLI.)
Genus Parypha
P. crocea. This is one of the most beautiful of the tubularians. It has a large, drooping head on a stem three to [pg124] four inches long. It is bright red in color, and from the center of the circle of tentacles the reproductive zoöids hang in a cluster, like a bunch of grapes. It does not liberate swimming-bells. It is found in bunches on piles of wharves and bridges, in brackish water, on the eastern coast as far south as Charleston, South Carolina.
Corymorpha pendula. See Plate XLII.
Genus Hybocodon
H. prolifer. One of the largest tubularians, somewhat resembling Parypha crocea. It is deep orange in color, and the head is erect on a long stem. The reproductive zoöids are in a cluster in the center of a double row of tentacles, and resemble a basket of fruit. It grows singly, or in groups of two or three, in shaded tide-pools, which are protected from the surf, and in which the water is very pure. It is found on the Massachusetts coast, but is not common. This species liberates swimming-bells. (Plate XLI.)
Genus Pennaria
P. tiarella. The branches are arranged alternately and at right angles to a central stem or axis; they taper, being shortest at the top and bottom of the stem. The zoöids are red in color, and are arranged along the upper side of branches at considerable intervals. The stems are black and beaded, being constricted at intervals. The zoöids have two rows of tentacles, the upper ones on a small proboscis. From the lower part of the proboscis deep, bell-shaped bodies, which eventually become swimming-bells, are developed. The species is found on rocks and eel-grass along the whole eastern coast.
P. gibbosa. A species similar to P. tiarella, found on the coast of Florida.
Genus Bougainvillea
B. superciliaris. This hydroid is found in tide-pools on the New England coast, growing in clusters, about two inches high, attached to rocks or to mussel-shells. The stem is very slender, and branches. It is red in color. The medusæ which it liberates are found in great numbers in the spring. The tiny swimming-bells are nearly globular. The tentacles are long, are arranged in four clusters on the margin, and extend in every direction. The manubrium is yellow and short, and the mouth is concealed by four clusters of short tentacles. On these oral tentacles [pg125] the eggs of the animal are produced. In its habits it is sluggish, often remaining in one position for several days.
The hydroids which have an open, bell-shaped cup at the termination of a short, stalk-like stem, or branchlet, are mostly campanularians. This division embraces jellyfishes of different families. Many medusæ cannot be referred with certainty to the hydroids from which they sprang, and the medusa-buds of many of the hydroids have not been noted.
Genus Obelia (Plate XLII, A)
O. commissuralis. This is a delicate, much-branched hydroid, five to six inches long, found at low-water mark in tide-pools, attached to stones and seaweeds, along the rocky shores from Nova Scotia to South Carolina. Its branches are arranged spirally and spread nearly at right angles to the main stem, and the main branches subdivide in a similar manner. Every interval of the stem has a slight curve, and at the base of every branch there are four or five rings. The ultimate branches, or pedicels, bear at their ends bell-shaped cups which have even edges, but are twelve-sided and slightly incurved. The pedicels are ringed for the whole length. The reproductive cups on short ringed pedicels are larger than the others, and occupy the angles of the branches. These cups are constricted and again expanded at the apex, forming an urn-like top.
Genus Eucope
E. diaphana. This species is often abundant on the fronds of Laminaria washed ashore, and also on Rhodymenia and Fucus. It has a creeping base, zigzag in form, but keeping a straight course, and in its branching often forming a network over the surface of the flat fronds. At each angle of the creeping stems rises a pedicel about an inch high, which inclines in the direction of the stem and terminates in a zoöid-cup similar in form to that of Obelia. The medusa which this hydroid liberates is called Thaumatias diaphana. The swimming-bell is very shallow and thin, turning inside out at almost every pulsation. The tentacles are numerous and rigid like stiff hairs. This little medusa is very active and is abundant. The species is found from Long Island Sound northward.
Genus Oceania
O. languida. This medusa is one inch in diameter and one half of an inch high, and is so delicate and transparent that it is hardly visible except in its outlines. In its early stages it is nearly spherical and has no tentacles; later the disk flattens and has from thirty-two to thirty-six [pg126] tentacles and numerous eye-spots. When disturbed it flattens its disk and folds together, leaving its tentacles sprawled in every direction. It is very languid in its movements, and often remains in one position for hours. These medusæ are found only in the hottest hours of the day, but are very plentiful then, shoals of them often stretching for miles, and so thick as to touch one another. Their habitat is the New England coast.
Genus Clytia
C. poterium. This hydroid is found creeping over seaweeds in tide-pools from Long Island Sound northward. The main stem is prostrate, or root-like, running over the body to which it is attached. The stems rise as do the tubularians. The sterile zoöids are on single stems about one quarter of an inch high. The stems are faintly ringed for their entire length, and at the top have a distinct ring, on which rests an open, bell-shaped cup, which is smooth around the rim. The reproductive zoöids are on very short pedicels, and the cups are long and cylindrical, with a wavy outline. (Plate XLII, A.)
C. bicophora. This species is found in the same places as the preceding, and is of about the same size. The long stems are more or less ringed and sometimes branched. The edges of the cups are notched. The medusa-buds are urn-shaped and ringed, and are on very short pedicels. (Plate XLII.)
The sertularians are distinguished by the horny cup, which is sessile—that is, set directly against the stem instead of being raised upon a stalk. They are among the most common objects of the beach, and, like the plumularians, are often mistaken for plants by the amateur collector and are gathered and pressed as seaweeds. They are found everywhere along the coast. They zigzag over the fronds of seaweeds or hang in fringes upon them, as well as upon rocks, stones, and shells. They well repay close examination with a glass. Every open cup bears a wreath of tentacles, which makes the branch a spray of stars. This is not an inappropriate comparison, for besides their starry shape some species emit a phosphorescent light.
| PLATE XLIII. | |
|---|---|
| Plumularian hydroid. | Aglaophemia struthioides. |
| Aglaophemia struthioides, magnified. | Plumularia falcata. |
Genus Sertularia
S. pumila. The most abundant of all the hydroids on the northeast coast is this species, which is found in profusion upon Fucus and other seaweeds, and mingled with them upon the rocks. It is easily distinguished from the campanularians because its zoöid-cups are close against the stems (sessile) instead of on stalks or pedicels. The stem creeps [pg127] over the fronds of seaweeds, often crossing and recrossing in a tangled mass. At short intervals the upright, straight branches rise to one inch or one and a half inches in height, and are more or less branched. All except the creeping stems are close set on each side, with cylindrical zoöid-cups which turn outward at the ends. The cups of the reproductive zoöids are not sessile; they are much larger than those of the nutritive ones and are urn-shaped. (Plate XLII, A.)
S. argentea. This is a beautiful species, common from New Jersey northward. It has a profusion of silvery branches on a dark stem. The colonies are often a foot or more long, and the branches at the top and bottom of the stem are shorter and fewer than those in the middle of the colony. The zoöid-cups are nearly cylindrical, pressed closely to the stem, nearly opposite or subalternate to one another, and end in pointed tips. The medusa-bud is urn-shaped, with two horns at the top. (Plate XLII, B.)
S. cupressina, the sea-cypress. This species is similar to S. argentea, but the main stem is thicker and longer, and the branches less crowded and less subdivided. The branches are arched or drooping, instead of straight, and gradually decrease in length at some distance from the lower and upper parts of the stem, giving a spire-like apex, the stem often continuing into a bare, branchless extremity. The zoöid-cups are tubular, not much narrowed or divergent above, and two-lipped on the margin. It is found from New Jersey northward. (Plate XLII, B.)
These hydroids are feather-like in the manner of branching, short lateral branches being arranged on each side of a long central stem. In some species the stems are naked below and resemble quills. The zoöid-cups are only on one side of the short branches. (Plate XLIII.)
Genus Aglaophemia
A. struthioides, the ostrich-plume. This species, which is found on the Pacific coast, is perhaps the most beautiful of the hydroids. It varies in size and color, but always suggests a small ostrich-plume. The zoöid-cups are arranged in a single row on one side of each short branch, and the main stem has a joint between each of the branches, which are placed quite close together. The rims of the cups have sharp-pointed teeth, and from the top emerge three tubular projections, which are called nematophores, and are supposed to be degenerate zoöids. At intervals a branch is replaced by a cylindrical body covered with nematophores, and in these the generative zoöids are developed. (Plate XLIII.)
Genus Plumularia
P. falcata (Johnston), or Hydrallmania falcata (Hincks). This species is found on shells and rocks near low-water mark from Long [pg128] Island Sound northward. It is from four to twelve inches high. The main stem is in long spiral turns, and at intervals has spreading plumose branches. The zoöid-cups are tubular and closely pressed against one another, and are ranged in rows on one side of the branchlets; the apertures of the cups are plain and oblique. (Plate XLIII.)
Genus Tima
T. formosa. A very delicate and transparent medusa; size one to two inches in diameter; bell conical; radial tubes four in number; manubrium long, hanging far below the disk; four frilled appendages diverging from the corners of the mouth; tentacles thirty-two; egg-sacs white and following the line of the radial tubes in undulating folds. This species is not very common; it is found on the New England coast.
Genus Zygodactyla
Z. groenlandica. Medusa seven to eleven inches in diameter; disk violet-colored and transparent; margin fringed with long, fine, contractile tentacles of a darker violet color; numerous radiating tubes; egg-sacs in slightly waved plates; manubrium hanging below the line of the disk and with a thin frilled membrane depending from it. Found north of Cape Cod in July.
The Trachymedusæ are characterized by their direct development, the egg of the jellyfish producing a medusa and not a hydroid colony.
Genus Trachynema
T. digitale. Size one inch to one and a half inches in height; rose-colored; the bell thin and hard, and conical at the top. The swimming is effected by contractions of the muscular velum (the band around the inner margin of the umbrella) instead of wholly by the bell. The tentacles are long and numerous, and are curled up when moving. The manubrium is long and has four expansions at the mouth. Eight egg-cases [pg129] hang in long pendent sacs from the upper part of the radial canals and reach nearly to the velum. Four garnet-colored eyes in club-shaped processes are prominent on the margin. The animal moves by jerks in straight lines.
The genus Millepora ("thousand pores"), which is the type of this order, is a colony of animals, like other hydroids, which secrete calcareous instead of horny coverings. It differs from true corals in that the members of a colony perform different functions, whereas in true corals each member of a community is a complete individual. It differs also in the arrangement of the stony partitions, which in Hydrocorallina are the outside coverings and connecting canals, but in true corals are vertical partitions inside the animal, between the inner and outer sacs, as explained on page 114.
Genus Millepora
M. alcicornis, elk-horn coral. This beautiful coral, which is abundant in Florida and contributes to the building of the reefs, rises in broad expansions, more or less lobed, and suggests by its shape the object for which it is named. The whole mass is porous, being traversed by innumerable canals. Its surface, although smooth compared with that of other corals, is covered with very minute pores, which are of two sizes. The larger ones are the gastropores, or stomach-pores, in which the nutritive animal lives; it has a cylindrical body, with four knob-like tentacles and a mouth. Placed more or less irregularly around the gastropores are smaller pores, the dactylopores (finger-pores), from which emerge slender mouthless processes, or dactylozoöids, with tentacles and stinging-cells. These seem to be the guard-polyps of the community. The cups occupied by the zoöids are shallow. As one animal dies, another succeeds it and builds a horizontal partition separating the new cup from the old one. Thus the stony mass increases in size by the progress of succeeding generations of zoöids. The living animal occupies only the outer, open space. (Plate XLIV.) [pg130]
This order of hydroids consists of free-floating communities and is one of peculiar interest, since it shows in a clear manner the special function of each individual member of the colony, and illustrates better than the foregoing, perhaps, the curious forms of animal life which this class presents.
Genus Nanomia
N. cara. This species is found on the New England coast. The members of the community are arranged along a hollow stem about three inches long which opens into every individual. At the top of the stem is a sac, or float; just below this is a group of swimming-bells which have no manubrium or mouth, and whose sole function is to provide locomotion for the community; and below these are three sets of zoöids, each having a triangular shield and tentacles. The tentacles are longer than the main stem. One of these last groups consists of the nutritive members, the mouths of the community, resembling manubriums of swimming-bells out of place. Each one has at the point of attachment a bunch of long, delicate tentacles having pendent knobs of lasso-cells. A second group, also with mouths, has shorter tentacles which are carried in spiral coils. The members of the third group have but one tentacle each and resemble the float at the end of the stem; presumably these drop off and produce new colonies. There are also on the lower part of the stem other reproductive members, which resemble the clusters of buds seen on [pg131] tubularian hydroids. These animals are pink in color and move through the water with a graceful swaying motion.
Genus Physalia
P. arethusa, the Portuguese man-of-war. This colony is perhaps the best-known one of the group, since it attracts much attention in Southern waters, and is also one of the most remarkable examples of an animal community. The most prominent part of the compound body is the float, an oblong pear-shaped bag, full of air, which floats on the surface of the water. Its color is bright blue, varying to rose. On the upper side of this air-vessel is a crest, or sail, and from the under side depend long tentacles, or streamers. Some of these tentacles are covered with stinging- or lasso-cells; some are the feeding zoöids, with flask-shaped bodies, and some, which look like bunches of grapes, are the reproductive zoöids. The tentacles in this curious cluster are all close together and hang from one side of the float, near the broader end. The longest are on the outside, which may be called the windward side, since they serve to keep the crest, or sail, before the wind; and when the wind is strong they stretch out to a remarkable length,—forty to fifty feet,—acting as anchors to keep the colony from being driven ashore. They also change its course by raising the pointed end of the float, thus forcing it to "come about." These long tentacles, ordinarily carried more or less curled up, are in bunches of two to four, and emerge from a common stem. Clusters of similar, but smaller, tentacles alternate with the larger ones, but grow somewhat nearer the pointed end of the float; these are purely locomotive organs. Next come two smaller sets of appendages, also of unequal size, which are the nutritive organs of the community. They are clustered together on a stem like the others. The appendages of the third kind are small, resemble bunches of grapes, and are scattered among the nutritive hydræ. These last are the reproductive zoöids of the community.
Genus Vellela
V. limbosa. This hydroid is abundant on the Florida coast. It has a bright-blue, flattened, oblong, bladder-like float, four to five inches [pg132] long, which, is divided into a number of concentric, communicating compartments. The margin of the float is entire, and a triangular sail extends diagonally across the top. On the under side is a single mouth on a manubrium, and surrounding it are a large number of short thread-like appendages having different functions. Some of them are feelers, others bear reproductive buds, and others have stinging-cells. Associated with Vellela is an allied hydroid called Porpita, which has no sail, and in company with these two is a jellyfish called Rataria, which is supposed to be the offspring of one or the other of them.
There is perhaps no marine animal which excites more wonder than the jellyfish. Its transparency, its graceful rhythmical movements, its long streaming tentacles, the variety and eccentricity of its form, and often of its color, attract attention, and one naturally desires to know something of its life-history. Jellyfishes are also called medusæ, because their long appendages suggest the locks of the Gorgon; acalephs, on account of their stinging or nettle-like properties; and sun-jellies, sea-blubbers, etc., because they float upon the surface during the warmest part of the day, when the sun is high. The name jellyfish is inappropriate, since the animal in no way resembles a fish except in the fact that it swims; but it is, nevertheless, the commonest name.
Jellyfishes vary in size from that of a pinhead to six or seven feet in diameter. They differ in the number, size, and position of the tentacles, the number of the radial canals, the form of the manubrium, the position of the egg-sacs, etc.; but the general plan of the internal structure is the same in all species. In shape they are compared to a mushroom. From the center of an umbrella-like top falls a central organ like the stalk of a mushroom. It is called the manubrium and is the mouth and stomach of the animal.
From the top of the manubrium radiate straight or branched tubes, which are connected with a canal which runs around the whole margin of the umbrella. Extending around the inner circumference of the disk in certain species (usually the hydroid medusæ), there is a horizontal shelf, called the velum, or veil, because it sometimes falls like a veil. [pg135]
From the margin of the umbrella depend the tentacles. There are little mineral deposits, like crystals, called lithocysts, disposed at intervals on the margin, and known also as marginal bodies, which are supposed to be eyes. In some species these lithocysts are inclosed in club-shaped bodies, and they are then called tentaculocysts, because they are like small tentacles. These, together with the nerve-fibers, are called the sense-organs; but to what extent jellyfishes can see and feel is undetermined. This is the first appearance of sense-organs in animals. Around the concave surface of the umbrella is a muscular zone, or zone of contractile tissue, by which the animal opens and shuts the umbrella and gets its locomotive power. The gonads, which are conspicuous from being more opaque than the rest of the body, are the egg- or sperm-sacs. They vary in form and in position.
The jellyfish is carnivorous, feeding on small organisms such as crustaceans and even fishes. The tentacles are invested with stinging-cells, as are also the frills about the mouth, when such occur. With these stinging-cells, which are in some species so powerful as to have been compared with an electric battery, the jellyfish benumbs its prey. The stinging properties are due to nettle-like threads contained in poison-cells. When these penetrate the flesh they produce a pain similar to that of an electric shock.
The food is taken into the manubrium by the square mouth at its free end, and is there digested. It is then sent as nutritive fluid through the canal system of the body, and ejected through small pores in the canal which surrounds the margin of the umbrella.
There are two sexes. The gonads of the female contain eggs; those of the male, sperms. The contents of the gonads drop into the central cavity and pass out through the mouth. The fertilized ovum is called a planula, and is a transparent sphere covered with cilia, by means of which it swims about for a time. At length it attaches itself to some object, and becomes in some species a branching colony (hydroid), in other species a strobila. The latter, as it grows, is constricted at intervals, and at maturity resembles a pile of inverted saucers with lobed edges. Each of [pg136] these saucers is finally detached, and when liberated is called an ephyrula, and becomes a jellyfish. Thus its cycle of life is complete. There are some species which, having no hydroid or strobila state, mature without alternation of generation (metagenesis).
The term of life of the jellyfish does not exceed one year. Even the giant Cyanea attains its immense growth in six months. It starts in the spring as an ephyrula, not more than one half of an inch in diameter, and when it dies in the autumn is often six to eight feet in diameter. The bodies of jellyfishes are ninety-nine per cent. water, and the dead ones thrown upon the beaches by the autumn storms rapidly disappear, leaving no traces behind.
The powerful stinging-cells with which the large medusæ are armed make them formidable enemies, and it is probable that some deaths by drowning are caused by swimmers encountering them and becoming paralyzed by them.
Genus Lucernaria