M. pilosa. An incrusting variety, composed of a single layer of cells spreading irregularly over the surfaces of stones, shells, and seaweeds, sometimes completely covering the fronds of Chondrus crispus, Phyllophora, Rhodymenia, and other algæ. The cells are membranaceous, with a calcareous rim, and have one long hair and several small ones surrounding the large roundish apertures. It is found in abundance on the shores from Long Island Sound to the Arctic Ocean.
M. lineata. Cells oblong, crowded, closely adherent; slender spines on edge of aperture, which bend over and meet across it; incrusts rocks and shells in broad, thin, radiating patches; cells [pg197] much smaller and narrower than those of M. pilosa; easily distinguished from that species by its manner of spreading and the absence of the one long hair. Common from New Jersey northward.
M. tenuis. Common on pebbles, often covering their whole surface with a lace-like incrustation of very small oblong cells having a three-lobed aperture. Found in Long Island Sound and north to Cape Cod.
Genus Escharella
E. variabilis. This species forms calcareous incrustations on shells or pebbles. The layers are thin, but eventually overlap one another, and the incrustation sometimes becomes an inch thick and resembles coral. The color of the living animal is dull red. The species ranges from Cape Cod to South Carolina, and is abundant in Long Island Sound.
Genus Mollia
M. hyalina. Cells subcylindrical, irregular, transparent, smooth, and more or less oblique. The species forms small circular disks on algæ in tide-pools.
Genus Cellepora
C. scabra. It forms branching, coral-like masses on slender red algæ.
C. ramulosa. Branches cylindrical, rough, dividing in a forking manner, spreading, two to three inches high, calcareous; cells urn-shaped, irregularly arranged, apertures contracted, long spine on the outer edge. It is found in deep water attached to shells, and in tide-pools, where it grows chiefly on Sertularia and other hydroids and on slender red algæ. It ranges from Long Island Sound to Greenland. (Plate LI.)
C. pumicosa. Cells urn-shaped, irregularly crowded together, forming a conglomeration of porous, brittle masses, not exceeding an inch in breadth, usually round when small, oblong and knobbed when large; spine on outer margin of aperture. It incrusts stones and the stems of Fucus. (Plate LI.)
Suborder CTENOSTOMATA
Genus Alcyonidium
A. ramosum. Twelve to fifteen inches high; much branched; branches smooth, cylindrical, one third of an inch in diameter, usually crooked; branches in a forking manner; color rusty-brown. Abundant in shallow water, attached to rocks, from New Jersey to Cape Cod.
A. hirsutum. A species common on seaweeds, sometimes completely covering them, forming dirty, straw-colored, thick, cartilaginous crusts covered with numerous conical papillæ. These are the cells, or zoœcia; they are surrounded with spines. Circular yellow spots occur over the surface, which are clusters of eggs. A good pocket-lens will show the cells, laid in beautiful order, and perhaps the extended tentacles. Found from Long Island Sound northward. [pg198]
A. hispidum. One of the most common species found incrusting the fronds of Ascophyllum at low-water mark. It forms fleshy, brown, soft crusts of moderate thickness, and has spines scattered over the otherwise smooth and glistening surface. The cells are inconspicuous, and each one has five long rigid bristles. Habitat, Long Island Sound to Greenland.
A. parasiticum. It forms thin, earthy crusts on algæ and hydroids. The surface is porous; the cells are distant and arranged irregularly, and seem as if composed of sand cemented with mud.
Genus Vesicularia
V. dichotoma (Valkeria pustulosa). This species grows in clusters of crowded slender stems, which branch in a forking manner, the branches dividing in different planes, making a tree-like form one to three inches high. At the points where the branches divide is a dark, opaque substance, and at these points also the cells are crowded in clusters of spiral rows and are greenish-brown in color. The dark spots are in marked contrast to the white translucent substance of the rest of the stem, giving a spotted aspect to the whole. (Plate LI.)
V. custata. Delicate, thread-like, jointed stems, with slender opposite branches; cells small and elliptical, arranged mostly in clusters. Found creeping like a small dodder-plant over other polyzoans, hydroids, and seaweeds.
Genus Pedicellina
P. americana. A very small species. Club-like zoœcia rise from slender, white, creeping stems; tentacles roll up instead of retracting into the cups. Found on hydroids, other polyzoans, and algæ.
Abo´ral surface: The side opposite the mouth.
Ambula´cra: Tubular feet used in moving.
Ambulacral zones: The five areas containing the rows of tube-feet.
Ampul´læ: Reservoirs or vesicles at the base of the tube-feet.
Auricula´ta: The larval stage of holothurians.
Bipinna´ria or Branchiolaria: The larval stage of starfishes.
Dorsal surface: The back of the animal, generally, but not necessarily, the upper side.
Exoskeleton: Outside framework or support, differing from a true skeleton which lies inside the body.
Interambulacral areas: The five areas between the ambulacral zones.
Larva: The animal in a stage of development from the time it leaves the egg until it reaches the complete form of the species.
Madreporic plate: A sieve-like plate of carbonate of lime.
Oral surface: The side on which the mouth is placed.
Ossicles: Calcareous plates which cover the body and form the exoskeleton.
Pedicella´riæ: Small spines which have divided ends, like scissors or forceps.
Plu´teus: The larval stage of sea-urchins.
Po´lian vessels: One to ten sacs arising from the ring-canal.
Ring-canal: The canal around the mouth.
Spines: Processes which rise from the surface of the animal.
Stone-canal: A calcareous tube leading from the madreporic plate to the ring-canal.
Suckers: Sucking-disks on the ends of the tube-feet.
Ventral surface: The side opposite the dorsal surface.
Water-vascular system: A series of canals which conduct water through the animal. [pg202]
The echinoderms, although their shapes are very unlike in the different classes, have the same general internal structure, and also other features which place them together in one group. They are radiates (page 113) of the highest type; they have an exoskeleton; and many of them are beset with spines, from which the name is given. They have locomotor organs, the ambulacra, and a water-vascular system peculiar to themselves. Some have the very strange power of casting off, and developing again, parts of the body.
The development from the egg to the adult is remarkable, and of unusual interest to naturalists. The larval stage of echinoderms is so unlike the mature animal that for a long time the larvæ were thought to be another class of animals, and therefore were given the names which they still retain.
In observing echinoderms it will be seen that the dorsal part is carried uppermost by some, such as the starfishes and sea-urchins. In the former the back is broad and extended; in the latter it is curved and contracted. But in crinoids the back is carried downward and is extended like a stalk; in the sea-cucumbers (holothurians) the dorsal and ventral surfaces are parallel with the long axis of the cylindrical body, instead of being on a plane with the mouth and excretory opening. Owing to the unusual positions of the dorsal and ventral surfaces, the terms oral (mouth side) and aboral (side opposite the mouth) are generally used in describing these species.
The surface of the body is divided definitely by the ambulacra, the ambulacral zones, and the interambulacral spaces, but differently in the different classes. The movement of the animals is [pg203] effected by means of the tube-feet, or ambulacra. These have suckers, which are attached or released by the power of the water-vascular system. The madreporic plate, which can be distinctly seen in starfishes at the angle of two of the arms, is a calcareous porous plate which opens into the stone-canal. This canal passes through the body and opens into a circular tube which runs around the mouth. A system of canals extends from this circumoral tube, or ring-canal, and eventually one canal opens into each tube-foot. Water, entering by the madreporic plate, passes through the various canals and into the tube-feet, which it distends. When the feet are pressed against a substance and the water is withdrawn, a vacuum is formed, which causes the disk-ends of the feet to act like suckers. When the feet are again flooded the hold is relaxed. Thus the animal drags its body along with a slow, gliding motion. The madreporic plate, being calcareous, is said to act as a filter, purifying the water as it passes into the body, which it furnishes with oxygen as well as with locomotive power.
A curious organ found in the Echinoidea (sea-urchins) is called Aristotle's lantern. It is a complicated arrangement, not fully understood, having forty parts, and is connected with the alimentary system. One of the parts is the mouth, over which five teeth project. These teeth grasp and grind the food. The exoskeleton is composed of plates of carbonate of lime, called ossicles, which cover the surface of the body. The ossicles are of various sizes and shapes, and are one of the determining features in classification. They may be scattered spicules of lime, separated plates joined by connecting rods, or overlapping plates; or they may be fitted together so as to form a continuous shell. The starfishes are usually carnivorous, the sea-urchins usually vegetarian, while the holothurians, after the manner of worms, take in sand and mud, deriving their nourishment from the organic particles contained in them. All echinoderms live in the sea. [pg204]
These animals are named from their star-shaped outline. Some have five long, spreading arms diverging from a small disk, which is hardly more than their point of union; others have a large body with short arms, which are like angular projections of the body, giving a pentagonal shape. Some genera have a greater number of arms than others; Solaster has eleven to thirteen, and Heliaster (the sun-stars, found on the west tropical coast) has thirty to forty.
The mouth of the starfish is in the center of the ventral side. A ventral or ambulacral groove extends through each arm. In these grooves the ambulacra, or tube-feet, are arranged in rows. The ambulacra are hollow, cylindrical bodies, each of which has a sucker at one end, and is connected at the other end with a little globular body, the ampulla. The ventral groove forms a ridge in the body-cavity. It is called the ambulacral zone, and is formed by a double row of elongated plates, which meet and form a raised line along the middle of the inside of the arms, and appear like ribs; between them lie the ampullæ. These plates are called the ambulacral ossicles. Numerous other ossicles extend over the surface, buried in the integument of the body, making a calcareous network, or exoskeleton. The ossicles are connected by muscles so that the animal is not rigid, but is able to bend the body and even creep through comparatively small places. The ossicles are covered with spines of two kinds. The very minute ones are the pedicellariæ, and have a jointed end which opens and shuts like a pincers. Their principal use seems to be to remove waste matter or other substances from the body, keeping it free and clean. The pedicellariæ are arranged, in some species, in circles around the [pg206] bases of the spines, and form groups over the dorsal surface. Examining the backs of different starfishes with a strong magnifying-glass, one will be surprised to see the varied grouping of the spines and pedicellariæ. At the extremity of each arm is a red eye-spot, which is sensitive to light, and a tube-foot, which has no sucker and which is believed to act both as an olfactory and as a feeling organ. The madreporic plate lies at the angle of two of the arms or rays. Water filters through the madreporic plate and passes by the stone-canal to the ring-canal around the mouth, and thence to the radial tubes, one of which extends through each arm and is connected by a branch with each tube-foot. At the base of each tube-foot there is a globular reservoir, or ampulla. A valve connects the tube-foot and ampulla. When the ampulla is contracted, water is forced into and distends the tube-foot; when the ampulla is distended, it withdraws the water from the tube-foot, thus acting like a suction-bulb. When water is sent into the tube-feet they are greatly extended and are then placed against an object. The water being then withdrawn through the ampullæ, a vacuum is formed, and the suckers are tightly attached. By the alternating movements of the ambulacra the animal is dragged slowly along and moves with a gliding motion. It does not deviate from a straight path, going over elevations or through depressions without turning aside; its body conforms to the irregularities of the road and never bridges over spaces. The stomach extends a little way into the arms, and a short intestine leads to the excretory opening in the center of the back. Starfishes are carnivorous and voracious, and are particularly [pg207] destructive to oyster-beds. The animal fastens itself to a bivalve, and protrudes a part of its stomach, with which it envelops its prey and slowly sucks it out of its shell. When feeding on small mollusks they take them into the stomach directly. Starfishes regain by natural growth parts of the body which may be lost. Sometimes the animal throws off an arm to escape capture, and self-mutilation also occurs where unfavorable conditions exist. Oystermen formerly were ignorant of the starfish's wonderful powers of regeneration, and were in the habit of cutting up those captured in their forks and throwing them overboard, thus increasing the number instead of destroying their enemies, as each arm with a piece of the body attached to it will, it is said, become a new individual. This tenacity of life makes starfishes difficult to destroy, and they are exceedingly plentiful in all seas. One naturalist speaks of seeing on the coast of Maine a bed of starfishes which extended several miles and covered the bottom so closely that he picked sixty individuals off a small stone. On the northern shores of the Pacific coast they abound in great variety, and some are of unusual size. Asterias gigantea of this region measures two feet across. Another species, Phyncopodia helianthoides, measures a yard in diameter, and has twenty or more arms. The most common species of the Atlantic coast are Asterias vulgaris and A. Forbesii; the former ranges from Long Island northward, the latter from Massachusetts to Florida. These are the species particularly destructive to oyster-beds. Asterias vulgaris sometimes grows to be fifteen inches in diameter; from this, which is perhaps the largest, are found starfishes of all sizes down to the very small Cribrella. They inhabit all varieties of bottoms, from low-water mark to deep water. They are not always abundant in the same place, but seem to move about.
Some oystermen believe that the starfishes get into masses like a ball and are rolled along by the tide. This idea comes from the fact that an oyster-bed may be free from them one day and the next be covered by these pests. To get rid of them the beds are swept over with a tangle, which is an iron bar holding swabs of raveled rope. The spines of the starfish are caught in the [pg208] tangle, and often hundreds are brought up in one haul and are then killed by steaming.
The anatomical system of the starfish is easily traced by cutting off, with scissors, the skin from the dorsal surface of the disk, and also cutting a slit down one of the arms. In the center of the body will be seen the upper part of the stomach, a small star-like spot, from which radiate five branches, which divide and lead into the much-ramified, plume-like organs which extend through the arms. These are the pyloric cæca, whose function is to secrete the digestive fluids. The much-folded stomach occupies the greater part of the central space, its large convolutions showing on top. The mouth connects with the stomach on the ventral side. By turning back the skin from the arm, small filiform processes can in some large species be seen; these are the dermal branchiæ, or breathing-organs, which may either project through pores in the skin between the ossicles or be entirely retracted. If the pyloric cæca be laid back, one can see the rib-like arrangement of the ambulacral ossicles, and also the ampullæ, which lie between the ossicles on each side of the ridge. If an ampulla be inflated and then pressed, the corresponding tube-foot will be seen to extend. To follow successfully the water-vascular system, beginning in the madreporic plate and extending through the stone-canal to the ring-canal around the mouth and thence through the arms, requires a strong glass and more skilful manipulation.
Genus Ctenodiscus (Plate LII.)
C. corniculatus. Pentagonal; about two and a half inches in diameter; the body flat, with wide marginal plates, giving a flat edge to the rather long, pointed arms; madreporic plate large; color greenish; ambulacra without suckers, seeming to be adapted to pushing through soft mud rather than dragging over hard surfaces. It lives in deep water on muddy bottoms and ranges from Massachusetts to Greenland. When dredged the animal is usually found to be filled with soft mud.
In this family there are only two rows of tube-feet; the rays end in sharp points and have large ossicles on the margins. [pg209]
Genus Astropecten
A. articularis. The body is flat and smooth; the rays sharply pointed and conspicuously bordered with marginal ossicles and fringed with short spines; two rows of ambulacral feet; color rich purple. Found in shallow waters of South Atlantic coast on sandy bottoms. (Plate LII.)
Genus Psilaster
P. floræ. Diameter four to five inches; arms sharply pointed and conspicuously bordered with large ossicles; body flat, with smooth surface, the ossicles seeming like mosaic; bright pinkish flesh-color. It occurs in deep water off the eastern coast as far south as New Jersey.
Genus Luidia
This genus of starfishes is celebrated for its peculiar habit of breaking in pieces when taken from the water, and therefore good specimens are almost impossible to obtain.
L. senegalensis. Twelve to fourteen inches across; arms long and narrow, tapering to a point and fringed on the edges with spines; surface rather smooth, showing the separation of the ossicles; color almost white, with dark line running through the center of each arm; usually nine arms. Found in shallow water on the coasts of Florida and the Gulf of Mexico.
L. clathrata. Four to five inches across; light-colored; with a fringe of spines on the margins of the arms, which taper to a point; two rows of ambulacra. It is found from New Jersey southward, and is one of the most common starfishes on the sandy beaches of North and South Carolina. (Plate LII.)
L. alternata. Upper surface purplish, with irregular yellowish bands on the arms; under surface yellow. Found in shallow water on the Florida coast.
Genus Mediaster
M. æqualis. Body flat; the five rays equal or exceed in length the diameter of the disk; margins have double row of large plates; surface covered with granules, which are easily rubbed off; bright red above, pale orange below; diameter four inches. Common on the coasts of Oregon and California.
Genus Hippasteria
H. phrygiana. This beautiful starfish lives in deep water, but is sometimes thrown up on the beach, and ranges from Cape Cod northward. [pg210] It has a large, pentagonal, bright-scarlet disk, five to six inches in diameter, with arms rather long and pointed. The upper side is raised and cushion-like, and is covered with blunt spines; the margins have larger spines of the same shape. The pedicellariæ are sessile and so long and large as to be easily seen with the naked eye. (Plate LII.)
Genus Pentaceros
P. occidentalis. Pentagonal; eight to twelve inches in diameter; ossicles a network of rod-like plates, with prominent spines at the points of union; body inflated in the center and running in wedge-like form to the ends of the rather long and narrow arms; the upper lines of the ridges have spines more prominent than those on the rest of the body; the spines of the larger specimens are more like large warts or small hemispheres. Found on the Lower California coast.
P. reticularis. This starfish is the largest species on our coasts. It is common in southern Florida close to the shore. The body is three to four inches thick, flat on the oral surface, but raised on the upper side, and tapers down in wedge-like form to the points of the short arms. The prominent blunt spines which cover its surface are arranged in a somewhat regular net-like order, and the sharp-edged margins have an even row of larger spines of the same form. The madreporic plate is nearly in the center of the disk, and is quite conspicuous, being bordered with a circle of small spines. Its color is dull yellow, and its size often a foot or more in diameter. It is the large starfish so often seen in curiosity-shops. (Plate LII.)
Genus Nidorella
N. armata. This very curious pentagonal species, found on the California coast, is five to six inches in diameter and red in color. The upper surface is raised and covered with thick, pointed spines one half of an inch to one inch long, arranged in lines, but not crowded. The ossicles on the margins are large; some of them are hemispherical and bear large spines. Smaller spines border the under edge of the margins. (Plate LIII.)
The starfishes of this family are pentagonal, with a large body and short arms. The disk is more or less elevated in the center, and the edges are sharp. There are two rows of ambulacra. The ossicles are notched and overlapping. The family is confined to temperate and tropical waters.
Genus Asterina
A. folium. Small, pentagonal, swollen (gibbous). Found on the Florida coast.
| PLATE LII. | |
|---|---|
| Ctenodiscus crispatus. | Astropecten articularis. |
| Luidia clathrata. | Hippasteria phrygiana. |
| Pentaceros reticularis. | |
A. miniata. Pentagonal; somewhat raised in center; about one and a half inches across; edges sharp. Found on the California coast close to shore. (Plate LIII.)
Genus Solaster
S. endeca. Dark red in color; nine to eleven arms, which are shorter than the breadth of the disk; diameter of disk about five inches; two rows of ambulacra; surface rather smooth. Common on the coast of Maine, and found from Cape Cod to Newfoundland below low-water mark.
S. decemradiata. This species is common on the North Pacific coast. It has ten arms, which are about twice the length of the diameter of the body, the whole body being about a foot across. (Plate LIII.)
Genus Crossaster
C. papposus. Twelve to fifteen arms, half as long as the breadth of the body; upper surface an open network of calcareous rods, or ossicles; at their points of union are club-shaped tubercles bearing tufts of smaller spines; concentric lines and spots of red and purple over the spiny upper surface; color light underneath. Common on the New England coast. (Plate LIII.)
Genus Cribrella
C. sanguinolenta. This species is abundant north of Cape Cod and is found of all sizes from half an inch to two inches in diameter and of various colors—purple, orange, red, yellow, flesh-color, etc. It has five round arms, generally about four times as long as the breadth of the disk. The ends are often turned upward. It is covered with crowded short spines, like little warts, which give it a nearly smooth surface. It has two rows of ambulacra. It moves with two of its arms turned forward, as if they were dragging the other three behind them. Unlike most species, the young are not free-swimming, but the eggs are carried around the mouth of the mother, and the young are retained there until they are capable of taking care or themselves. (Plate LIV.)
Genus Echinaster
E. sentus. This species occurs close to shore in the waters of Florida and is perhaps the most commonly observed species on the beach. It extends as far north as New Jersey. It is purplish in color, and about four and a half inches in diameter, with two rows of ambulacra, five arms, and a surface rough with spines. [pg212]
Genus Heliaster
H. multiradiata. Spines on the upper surface of rays in five rows, but close to the disk proper become reduced to one row. Found on the coasts of Lower California and Mexico. (Plate LIV.)
The Asteriidæ have the following characteristics: four rows of tube-feet; ossicles small and unequal; spines isolated or grouped; pedicellariæ of two forms, forceps-like and scissors-like respectively. They include the very common forms found on all beaches.
Genus Asterias
A. vulgaris. The common starfish of the Atlantic coast, from Long Island Sound to Labrador. It occurs at low-water mark and extends into deep water. It has five arms, which taper to a point. Large specimens measure fifteen inches across. The upper surface is rough, being covered with short spines, which are largest and thickest at the edges of the rays, and surrounding them are the pedicellariæ. The color varies from pink, yellow, and brown to purple.
A. Forbesii. The common starfish of the Atlantic coast, from Massachusetts Bay to the Gulf of Mexico. This species resembles very closely A. vulgaris, but can be distinguished from it by the madreporic plate, which is bright orange, while in the former it has the same color as the animal; also the arms are a little swollen at the base and terminate more bluntly. These two species are very destructive to oyster-beds, especially where their ranges overlap. It is computed that they destroy annually two hundred thousand dollars' worth of oysters. Vast numbers congregate where the feeding-ground is good, and move in long lines from place to place. The oystermen dredge over the beds and bring them up in thousands, then steam them or throw them on the shore above high-water mark. (Plate LIV.)
A. ochracea. The common starfish of the Pacific coast, from Sitka to San Diego. Five rays, each hardly twice as long as the diameter of the body; spines running irregularly over the surface, but forming a pentagon at the middle of the disk and inclosing the madreporic plate; diameter eight inches. It is very common near San Francisco on rocks at low-water mark. (Plate LIV.)
A. gigantea. Body very large and swollen; six rays, somewhat less in length than twice the diameter of the disk; aboral surface covered with numerous short, blunt, equidistant spines of uniform size and regularly distributed; spines contracted at the base and striated; diameter two feet. Found on the California coast.
The ophiurans, or brittle-stars, differ from the starfishes in having the arms quite distinct from the body. In starfishes the arms are extensions of the body, and the viscera extend partway into them, while in ophiurans the body is a central disk, with the five narrow arms attached to its margin. In form they suggest a spider, and sometimes are called sea-spiders. The name brittle-star is also descriptive, since they break off their limbs readily. The arms are narrow, taper to a point, usually curl at the ends, and have no ambulacral grooves, but are completely covered with bare calcareous plates (except in the first order, where they have a skin and no plates). The ambulacra are small spines without suckers, which emanate from the sides or margins of the arms and do not serve for walking. The madreporic plate is on the ventral surface, in one of the circular shields which surround the mouth. The alimentary canal ends blindly. The egg-sacs lie between the arms and open by slits on the under side, close to the arms, where they join the disk. Water flows in and out of these pouches, which are thought to have also respiratory and excretory functions. No eye-specks have been found, but they must exist, since the animal is sensible to approaching danger and quickly retreats. The ophiurans are more active than starfishes, moving by wriggling, and clambering with their arms. They are shy and hard to find, and it is difficult to capture a whole one, since they throw off pieces of their arms at the least alarm. Often they completely dismember themselves, an action which does them no permanent injury, since they reproduce lost parts. They are more commonly inhabitants of deep than of shallow waters, and are brought up from the bottom in dredges in great numbers. [pg215] Some species may be found near low-water mark under stones and in clusters of mussels, and often in seaweeds, thrown up from deep water, on the shore.
Genus Astrophyton (Plate LV.)
A. Agassizii. This very singular ophiuran is commonly called the basket-fish, from its resemblance to a basket when the tentacles are rolled up. Its body is covered with skin instead of calcareous plates. The body is thick and somewhat circular in form, with elevated radiating ridges on the upper side, and the skin is marked off in star-like divisions. From the margin of the body extend five arms, which at once divide in a forking manner; each section again divides, and this division in pairs (dichotomous division) continues until the ends of the arms have become very numerous and attenuated. The arms are carried curled up or straight at will. In moving, the animal seems to walk on these branches as if on tiptoe, and in this position it forms a kind of net which entraps prey. The arms and prominent parts of the disk are yellow, and the depressed or membranous parts brown. Astrophyton is six to eighteen inches in diameter. It is found off the northern New England coast.
Genus Ophiopholis