656 THE INVERTEBRATA 



gathered from the water to the mouth for food. The podia can only 

 serve for respiration and to increase the ciliated surface : they are not 

 prehensile, and it is said that only those around the mouth are sensory. 



Everywhere except in the grooves, the ectoderm is vestigial and 

 cuticulate like that of the Ophiuroidea. The dermis, which on the 

 oral side is merely leathery, contains on the aboral side a skeleton of 

 large ossicles. This consists of: (i) the centr odor sal \ (2) the small 

 rosette (formed by the fusion of five larval pieces known as basals), 

 which is internal and roofs a cavity, presently to be described, in the 

 centrodorsal ; (3) in each radius, three radials, of which the first is 

 usually not visible externally; (4) in each arm, a row of brachials; 

 (5) in each pinnule, a row of pinnularies', (6) in each cirrus, a row of 

 cirrhals, which are hollow. The ossicles of the appendages of the body 

 are movable upon one another by muscles. 



The alimentary canal consists of a short, vertical oesophagus^ a wide 

 stomach, curved horizontally around the axis of the calyx and bearing 

 two long diverticula and some low pouches, and a short intestine^ 

 which ascends to the anus. 



The perivisceral coelom (Fig. 459) of the calyx is traversed by 

 numerous calcified strands (trabeculae) . In the arms there are present 

 (i) a pair of subtentacular canals, (2) aboral to these, a genital canal, 

 (3) aboral to this again, a coeliac canal, which is derived from the right 

 posterior coelom of the larva. All these canals lead from the peri- 

 visceral cavity. It is said that there is a tiny perihaemal vessel in each 

 arm but no oral perihaemal ring. There is no genital (" aboral") ring 

 sinus. In the hollow of the centrodorsal ossicle lies what is known as 

 the chambered organ (Fig. 459, ch.on.). This consists of five radial 

 compartments, derived from the larval right posterior coelom; its 

 wall is richly nervous and constitutes the centre of the aboral or 

 apical nervous system. From the centre issue five interradial nerves, 

 which branch and form a complicated plexus (Fig. 460) with a co- 

 ordinating circular commissure, and from this plexus radial nerves 

 supply the arms and pinnules. Nervous prolongations of the cham- 

 bered organ also pass down the cirri. The whole of this system is 

 enclosed in the ossicles of the adult, but it originates from the wall 

 of the adjacent coelom. It controls the movements of the animal. If 

 it be destroyed they cease ; but the ectoneural system (which has the 

 same arrangement as that of a starfish) can be cut away without 

 affecting the movements. 



The axial organ lies in the axis of the body. Starting as a slender 

 strand in the centre of the chambered organ where the walls of the 

 chambers meet, and enlarging in the perivisceral cavity, it narrows 

 again orally, where it is continuous with a circular genital rachis. 

 From this again genital cords pass down the arms in the genital canals 



