Fig. 144. Skeleton of Synapta.
The calcareous particles imbedded in the skin of the genus Synapta are anchor-shaped spicules fixed to elliptical or oval plates, ([fig. 144]). The plates are reticulated and sometimes leaf-shaped, and the flukes of the anchors are either plain or barbed. All the anchors are fixed transversely to the length of the animal, lying with great regularity in the interspaces of the longitudinal muscular bands. Sometimes a thousand anchors are crowded into a square inch, each elegant in form, perfectly finished, and articulated to an anchor-plate, whose pattern as well as that of the anchor itself is characteristic of the species to which it belongs. In the Synapta digitata, which has four fingers and a small thumb on each of its twelve oval tentacles, the anchors are but just visible to the naked eye;[[36]] in all the other species they are microscopic. Besides the anchors, the skin of the genus Synapta contains innumerable smaller particles, ‘miliary plates,’ which are crowded over the muscular bands. The muscular system of the Synapta digitata is so irritable that, on being touched, it divides itself into a number of independent fragments, each of which keeps moving for a time, and ultimately becomes a perfect animal like its parent. Specimens of this Synapta have been found on the southern coasts of England and in the West of Scotland, but the genus is rare, although containing several species in the British seas; it is more common in the Adriatic; but they cannot be compared, as to size, with the great Synapta of Celebes, which is sometimes a yard in length, and is known among the natives as the Sea Serpent.
Fig. 145. Wheel-like Plates of Chirodota violacea.
The calcareous particles imbedded in the skin of the allied genus Chirodota are wheel-shaped when viewed with a microscope ([fig. 145]). One species is British, but they are mostly inhabitants of warm seas. In Chirodota violacea, a Mediterranean species, the skin is full of groups of broad thin hyaline wheels lying upon one another and connected by a fine thread. The wheels have five or six flat radiating spokes.[[37]] The wheels are exceedingly small in the Chirodota lævis, and are arranged in groups; in the C. myriotrochus they are imbedded in myriads, as the name implies.
Echinodermata Sipunculidæ.
The Sipunculidæ, which form the last order of the Echinoderms, consist of several genera of marine worm-shaped animals which burrow in the sand, and form a link between the Holothuridæ and the true sea-worms. They have no calcareous particles in their flexible skins, nor have they any tubular feet, or special respiratory organs, but a vascular liquid is kept in motion in the internal cavity by the cilia with which it is lined. The mouth of the Sipunculus is a kind of proboscis with a circular fringed lip and two contractile vessels, supposed to serve for raising the fringes. An alimentary canal extends to the end of the animal, turns back again, and the intestine ends in a vent near the mouth, so that the creature need not leave its burrow and expose itself to enemies in order to eject the refuse of its food. The locomotive larval zooids from the rose-coloured eggs undergo two metamorphoses; at last the young Sipunculus unites with the zooid, and no part is thrown off.
SECTION VII.
THE CRUSTACEA.
The Crustacea are free, locomotive, articulated animals, covered with a crust or external skeleton, and distinguished by having jointed limbs, and gills that fit them for aquatic respiration. They are male and female, and, though extremely diversified, they have a similarity in their general structure. Many are microscopic.