ORGANS OF SUPPORT, 



These two simple genera of acalepha present examples of 

 the thin, light, and delicate forms of the skeletons which 

 we find in almost all the floating marine invertebrata. 



V. Echinoderma. The skeletons of the animals of this 

 class are generally in the form of external crusts or shells, 

 covered with projecting spines. They are composed of the 

 carbonate, mixed with a small but variable proportion of the 

 phosphate of lime, and are hardened by animal matter. The 

 phosphate is always in a small quantity compared with the 

 carbonate of lime, but is more abundant in the solid shells of 

 the echinida than in the softer coriaceous and tuberculated 

 coverings of the stellerida. The skeleton of all these animals 

 consists of numerous detached or separate pieces, which protect 

 the interior viscera, give attachment to the organs of motion, 

 and generally give form to the whole body. The solid pieces 

 which compose the skeleton are for the most part in form of 

 calcareous plates, symmetrical in their shape and in their 

 arrangement, and which present considerable uniformity of 

 plan in their disposition throughout the diversified forms of 

 this class. The body most frequently presents a radiated 

 form in the animals of this class, the parts projecting in a 

 stellular manner from around a longitudinal axis, as is seen 

 in the various crinoid animals fixed by a jointed peduncle 

 and ramified above, and in the various forms of existing 

 stellerida, as the asterias, the ophinra, the euryale, and the 

 comatula, which are not fixed by a peduncle ; and we can 

 easily observe the same plan of structure in the more con- 

 centrated and globular forms of the echinida, as the scutellae, 

 the clypeasters, the spatangi, and the echini. 



The radiating portions of these animals are composed of 

 numerous rings or segments, like the trunk and members 

 of articulated animals, and each of these component seg- 

 ments is surrounded by numerous calcareous plates. One 

 of these radiated or stellular forms of echinoderma is seen in 

 the common asterias aurantiaca, (Fig. 7- A.) where there are 

 five rays or divisions of the body, the number most frequent 

 in this class. On examining the sides of these rays from 

 above,, as the animal is placed in the figure, we observe the 

 ends of large lateral plates (Fig. 7, A. a a a) which bound 

 the margins of all the rays. These plates are connected 

 with others which surround chiefly the sides and lower surface 



