626 



THE INVERTEBRATA 



among the bases of the epithelial cells — a nerve- net. The characteristic 

 ossicles of the dermis may be scattered, so as merely to impart a 

 leathery consistency to the skin, or united by muscles as a skeleton, 

 or firmly apposed so as to constitute an armour. Some of them usually 

 project as spines, over which the epidermis may presently wear away. 

 Pedicellariae (Figs. 433 C, 449) are sets of two or three spines ar- 

 ranged to bite together as pincers. They are of various types, often 

 complicated, but only occur in asteroids and echinoids. 



The alimentary canal differs greatly in the several groups. It is 

 axial in the Asteroidea and Ophiuroidea, coiled in the other classes. 

 It possesses various diverticula in different cases, but not large glands 



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Fig- 435- Representatives of classes of the Echinodermata, in their natural 

 positions. A, A starfish (Asteroidea). B, The shell of a sea urchin (Echinoi- 

 dea). C, A holothuhan. D, A sea lily (Crinoidea). 



like those which are common in other phyla. The anus is lacking in 

 the Ophiuroidea and a few asteroids, and when present is more or less 

 excentric except in the Holothuroidea. 



The coelom of the adult is present as several distinct systems of 

 spaces, of which the following are the most important: (i) the large 

 perivisceral cavity in which lie all the principal viscera; (2) the peri- 

 haemal system, consisting of a radial vessel (in asteroids divided longi- 

 tudinally by a vertical septum in which lies the principal "blood" 

 strand) along each radius, and a ring vessel around the mouth, all 

 lying immediately above the main nerve cords; (3) the aboral sinus 

 system enclosing the genital rachis and gonads (see below); (4) the 

 water vascular system (Fig. 436), which lies above the perihaemal 

 system, and consists of a ring around the mouth, a tube, known as the 

 stone canal because its wall is frequently calcified, leading to an open- 



