The genus Comatula are star-fishes, believed to have alternately a fixed and a free state. Mr. J. V. Thomson discovered that the Pentacrinus Europæus is merely the fixed state of a Comatula. These star-fishes have pairs of pinnæ placed at regular distances along their long-jointed rays, and in the pinnæ sacs containing eggs are placed as far as the fifteenth or twentieth pair. The eggs yield active ciliated larvæ, which attach themselves in the form of flat oval disks to corallines and sea-weeds. By degrees they develop a stem, about three-fourths of an inch high, with twenty-four distinct joints. Its expanded top bears five sulphur-coloured bifurcating rays with their pinnæ and dorsal cirri. A mouth is formed in the centre with its tentacles, and a lateral prominent vent. The actual change of a Pentacrine into a Comatula has not been seen, but as the small Pentacrinites disappear in September, at which season the Comatulæ appear, it is believed that when full grown the top of the fixed Pentacrinite falls off and becomes a Comatula, which swims backwards with great activity by striking the water alternately with its long rays. The Pentacrinus caput-Medusæ, which is fixed by its stem to sea-weeds and zoophytes, forms a most beautiful object for the lower magnifying powers when viewed in a fluid by a strong refracting light.
Echinodermata Echinoïdea.
The family of Echinidæ, commonly known as Sea-Eggs or Sea-Urchins, have a beautiful but complicated structure. The calcareous shell of an Echinus is a hollow spheroid with large circular openings at each pole. In the larger of the two, called the corona, the mouth of the animal is situated; in the lesser circle the vent is placed. The spheroid itself is formed of ten bands extending in a meridional direction from the corona to the lower ring; that is, from one polar circle to the other. Each band consists of a double row of pentagonal plates increasing in size from the poles to the equator, nicely dovetailed into one another, and the bands are neatly joined by a zigzag seam. Every alternate band is perforated by a double series of minute double holes for the passage of the tubular feet of the animal. The five perforated or ambulacral bands have rows of tubercules parallel to the series of feet holes, supporting spines movable in every direction. The five imperforated bands are characterized by a greater number of spines, but there are none within the polar circles. The spines may be long rods, or merely prickles, or stout, club-shaped bodies, according to the genera.
Fig. 138. Section of Shell of Echinus. a, portions of a deeper layer.
The microscopic structure of the shell of the Echinus is everywhere the same; it is composed of a network of carbonate of lime, with a very small quantity of animal matter as a basis. In general, the network extends in layers united by perpendicular pillars, but so arranged that the open spaces, or meshes, in one layer correspond to the solid structure in the next.
The spheroid of the Echinus is covered with spines, and both outside and inside by a contractile and extensile transparent membrane, which supports the shelly plates at the poles, and dips between the bands but does not penetrate them. Its extensile nature admits of the addition of calcareous matter to the edges of the plates when the animal is increasing in size. The membrane lining the interior of the shelly globe is tough; it encloses the digestive organs, and forms a muscular lip to the mouth, which is armed with five triangular, sharp-pointed, white teeth, and surrounded by five pairs of pinnate tubular tentacles. The outer margin of the lip is fringed with a circle of snake-headed pedicellariæ visible to the naked eye.
The five teeth, whose sharp tips meet in a point when closed, are triangular prisms, the inner edge is sharp and fit for cutting. Each tooth is planted upon a larger triangular socket, two sides of which are transversely grooved like a file, and as these two sides are in close contact with the sides of the opposite socket, the food when cut by the small teeth is ground down by the sockets, and a salivary secretion finishes the mastication. The sockets of the teeth are connected by ten additional solid pieces, placed two and two between them, which completes the pyramidal apparatus called Aristotle’s lantern; it consists of forty solid calcareous pieces arranged in fives, and moved by forty muscles attached to five calcareous ridges, and five arches near the internal edge of the corona.
Five pairs of these muscles when acting together protrude and retract the teeth; when acting separately they draw them to one side or to the other; five pairs separate the five teeth, five pairs shut them, and the remaining five pairs work the bruising machine. The masticated food passes through a short gullet into the stomach, where it is digested, and the indigestible part is carried by an intestine to the vent in the smaller polar circle.
The smaller polar circle is formed of ten triangular plates, five are attached to the bands containing the feet holes, and five to the intermediate bands. The last five are perforated, and are the reproductive plates: the other five are also perforated for the discharge of the liquid that moves the tubular feet, and which, after having circulated in the body, is no longer of use. In five of these polar plates there are red specks, the rudiments of eyes, the only organs of sense these creatures seem to possess except that of touch and probably smell. The nervous system is a slender, equal-sided pentagon round the gullet, from the sides of which five nerves are sent to the muscles of the mouth, and others, extending along the ambulacral or feet bands, end in nerve-centres under the eye-specks.