2 88 ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



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



aer, 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 



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



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



iht the fluid, in addition to the amoebocytes, receives various 



chemical products of the metabolism of the surrounding tissues. 



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



us that of Astcrias while differing in details. Thus the circum-oral ring 



ten attached to it even amongst Asteroids five " Polian vesicles " 



looking like large ampullae. In Holothurians (Fig. 116, D) the tube- 



nd to degenerate over most of the body but a circle round the 



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



fashion (Fig. 116, D, /) : they are used for collecting food and passing it 



in to the mouth. 



The nervous system of the Echinoderms is on the whole remarkable 

 tor 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 

 H 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 

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

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

 rent nil nervous system can easily be displayed in a starfish by pressing 

 apart the tu he-feet from the centre towards each side of the ambulacral 

 and then scraping them off. By examining a thin transverse 

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

 nervous strands are simply ectodermal thickenings and that they retain 



jirimitive superficial position (Fig. 118, r.n), 



In other K hinoderms, except the Trinoids, the nerve strands lose 

 ;>rr!inal position. What corresponds to the ambulacral groove 

 lie.omes as it were closed in and converted into the epineural space and 

 the i .! is found on the roof or inner wall of this. 



A remarkable peculiarity of Kchinoderms is that in addition to the 



ordinary nervous system derived from the ectoderm they possess an 



QC developed from the coelomic lining. In the transverse 



through the arm of Asterias (Fig- "8) there are seen between 



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



