580 THE INVERTEBRATA 



centrodorsal ; (3) in each radius, three radtals, 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. 434) 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. 434, 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. 435) 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 

 aflPecting 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 

 and so reach the pinnules, where they enlarge into gonads. The genital 

 cells are dehisced by rupture when ripe. The lacunar system has an 

 oral ring and " vessels " from this to a plexus on the stomach and to the 

 lacunar tissue of the axial organ. It is doubtful whether radial vessels 

 are present. 



The water vascular ring closely surrounds the mouth, and from it 

 numerous stone canals open without madreporites to the perivisceral 

 cavity, which in turn communicates by many isolated pores, lined by 

 cilia, with the exterior. This arrangement is due to the fusion of the 

 larval axial sinus with the perivisceral cavity and subsequent multi- 



