[Illustration: 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. The Crustacea constitute ten orders, many genera, and innumerable species. The Decapods, or the ten-footed order, are by far the most complicated in organization. They have prominent eyes, movable on jointed stalks, antennæ, gills in a cavity on each side of the throat, a mouth opening into a digesting apparatus, a heart, liver, circulation of the blood, and a nervous system, and are therefore animals of a higher grade than any that have come under consideration.