202 



PHYLUM ECHINODERMATA. 



Some or all of the corpuscles in the water-vascular fluid of 

 Ophiactis virens are coloured red with haemoglobin. 



The axial organ has the same structure as in Asteroids. It 

 is found on the side of the stone-canal turned away from the 

 mouth (Fig. 145). It is in relation witli a section of the body 

 cavity (Fig. 145, 7), which is quite unconnected * with the 

 ampulla (axial sinus) and is probably the diverticulum of the 

 left posterior body-cavity of the larva which is formed in the 

 invagination of the primitive germ-cells (MacBride). 



The axial organ (Fig. 

 145, 8) is continuous with 

 the generative rachis, 

 which is contained in the 

 aboral sinus (Fig. 144, PH) 

 16 (a portion of the left larval 

 coelom as in Asterids) and 

 takes a somewhat peculiar 

 course round the disc. 

 Radially it lies in the 

 aboral part of the disc 



FIG. 145. Diagram of a vertical section through the j lp f wppn fV,p stomach cae- 



madreporitic interradius of Ampkium squamata, Between 1 



showing the relations of the axial organ, stone- ,,, Qrir l f] 1P nrmpr in- 



canal and neighbouring sinuses (after MacBride). ' 



1 circumoral water-vascular vessel; 2 stone- f pmlTrlf vnf wViprpn in pirli 



canal; 3 ampulla (axial sinus) ; 4 pore-canal ; tegument, Wiiei 



5 closed sac which appears to represent the right infpwa/line if r1ir> rlnwn 



hydrocoel; 6 aboral sinus in its ventral position; mtCliaC 



7 sinus derived from the left posterior coelom and U p 4- wppn f! 1p etnmaoli rappa 



sometimes called the axial sinus ; 8 axial organ ; DetWCen W16 



9 genital rachis; 10 genital bursa ; 11 wall of f^.,, r ,1 Q f| 1p Inwpr <5iirfflf>p 



stomach ; 12 perioesophageal sinus ; 13 inter- OWarClS U16 



radial muscle; 14 nerve ring; 15 teeth; 16 /f^: CT ~\ AA\ Tf liac Vippti 

 mouth ; 17 oral surface of disc. 



suggested that this course 



is due to the fact that, in the interradii, structures which were 

 originally on the upper side of the disc have moved on to the 

 lower surface ; as an instance of this may be cited the water- 

 pore which in the young form is dorsal, but in all adults has 

 passed on to the ventral surface. 



The perihaemal system consists of a circumoral canal 

 which is prolonged into a radial tube in each arm on the upper 

 side of the nerve cords (Figs. 144, ePH ; 140, 10). There does 



* Cuenot, who thought that the two are in communication, appears to 

 have been in error. His mistake is reproduced, together with an errone- 

 ous figure, on p. 820 of the section on Echinoderms in Bronn's Thierreich 

 (1900). 



