564 INVERTEBRATE MORPHOLOGY. 



palce angulares (Fig. 260, p), whose bases generally fuse to 

 form a supporting plate, the torus angularis (T). 



Spines developed in connection with the dermal skeleton, 

 leaving out of consideration the oral angle-pieces, may be en- 

 tirely wanting, but in many forms they are borne in vertical 

 rows upon the adarnbulacrals, and are usually movable. In 

 a few forms, especially those inhabiting rocky bottoms, pe- 

 culiar hooked spines are situated on the oral surface of the 

 arms towards their extremities, and seem to serve an adhe- 

 sive function. Pedicellarue are absent, except in Astropliyton 

 and allied genera. 



The coelom (Fig. 260, c) is of comparatively slight extent, 

 the cavity of the disk being largely occupied by the digestive 

 tract, and that of the arms by the ambulacral ossicles. In the 

 disk the cavity is traversed by numerous bands which ex- 

 tend from the body-wall to the wall of the digestive sac, 

 and from the wall of the oesophagus a membrane extends 

 outwards and orally to be attached to the peribuccal plates, 

 forming a septum (Fig. 260, s), enclosing a cavity surround- 

 ing the oesophagus, the peripharyngeal space (ps), which is 

 completely separated from the rest of the coelom. In Opldo- 

 thrix and some other forms a second septum occurs parallel 

 to the one just mentioned, so that the peripharyngeal space 

 is double. The coelom of the arms consists of two portions, 

 one lying on the aboral and the other on the oral side of the 

 series of ambulacral ossicles. The aboral cavity is expanded 

 laterally so as to partially surround the ossicles, but this lat- 

 eral portion is traversed opposite each ossicle by a calcareous 

 lamella, and is thus separated into a series of chambers 

 which open into the undivided aboral portion, termed the 

 aboral or dorsal canal. An axial sinus, standing in close re- 

 lationship to the ovoid gland, exists, but presents some fea- 

 tures not found in the Asteroidea. It consists in Ampldura 

 sguamata (Fig. 259) of three distinct portions completely sep- 

 arated from one another ; one of these is the so-called am- 

 pulla (am) of the stone-canal ; the second (s) lies in close 

 relation to the ovoid gland, which is developed on its axial 

 wall ; while the third is comparatively small, and is associated 

 with the genital cords (f/r), and the mass of cells in the ovoid 



