288 ZOOLOGY FOR MEDICAL STUDENTS chap. 



The fluid of the hydrocoele is, as will have been gathered, nearly pure 

 sea water, but it is inhabited by amoebocytes which are budded off into 

 it by nine little pockets of the inner wall of the circum-oral ring known 

 as Tiedemann's bodies : these are arranged in pairs between each two 

 rays except that the stone-canal occupies the position of one of them. 

 No doubt the fluid, in addition to the amoebocytes, receives various 

 chemical products of the metabolism of the surrounding tissues. 



In other Echinoderms we find a hydrocoele of the same general type 

 as that of Asterias while differing in details. Thus the circum-oral ring 

 has often attached to it — even amongst Asteroids — five " Polian vesicles " 

 looking like large ampullae. In Holothurians (Fig. ii6, D) the tube- 

 feet tend to degenerate over most of the body but a circle round the 

 mouth are greatly enlarged, and in some cases branched in a tree-hke 

 fashion (Fig. ii6, D, t) : they are used for collecting food and passing it 

 in to the mouth. 



The nervous system of the Echinoderms is on the whole remarkable 

 for its very primitive character. Towards the inner surface of the soft 

 ectoderm there exists a diffuse nervous network like that of a coelenterate 

 which serves to link together the various parts of the surface into a 

 coherent whole. In a Sea-urchin it is easy by rotating a cork-borer or 

 apple-corer against the hard test to make a circular break in the con- 

 tinuity of the network and then it is seen that the spines within the isolated 

 area no longer move in co-ordination with those outside it. Special 

 concentrations of the network, constituting an incipient central nervous 

 system, form a circum-oral nerve ring (Fig. 117, n.r) and a radial nerve 

 (Fig. 117, r.n) running out from this along each ambulacrum. This 

 central nervous system can easily be displayed in a starfish by pressing 

 apart the tube-feet from the centre towards each side of the ambulacral 

 groove and then scraping them off. By examining a thin transverse 

 section of the arm through a microscope it is seen clearly that the central 

 nervous strands are simply ectodermal thickenings and that they retain 

 their primitive superficial position (Fig. 118, r.n). 



In other Echinoderms, except the Crinoids, the nerve strands lose 

 the superficial position. What corresponds to the ambulacral groove 

 becomes as it were closed in and converted into the epineural space and 

 the nerve strand is found on the roof or inner wall of this. 



A remarkable peculiarity of Echinoderms is that in addition to the 

 ordinary nervous system derived from the ectoderm they possess an 

 accessory one developed from the coelomic lining. In the transverse 

 section through the arm of Asterias (Fig. 118) there are seen between 

 the ectodermal radial nerve on the one hand and the radial canal of the 



