i86 



PHYSIOLOGY OF THE INVERTEBRATA. 



existence of the anal ring, and of an extensively ramified 

 system of canals, connected with it and with the nenral 

 canals." But according to Prof. Huxley, " the facts, as they 

 are now known, do not appear to justify the assumption that 

 these canals constitute a distinct system of blood-vessels." 

 Prof. Huxley doubts the special circulatory function of the 

 neural canals, and he does not consider that the sinus which 

 accompanies the madreporic canal is in reality a heart. He 

 states that this sinus and canals " are mere sub-divisions of- 

 the interval between the parietes of the body and those of 



Fig. 36,— Circulation in Echinodermata (Sea-urchin). 



the alimentary canal, arising from the disposition of the. 

 ambulacral vessels and that of the walls of the peritoneal 

 cavity ; both of which, as their development shows, are the 

 result of the metamorphosis of saccular diverticula of the 

 alimentary canal, which have encroached upon, and largely 

 diminished, the primitive perivisceral cavity which exists in 

 the embryo. The peritoneal cavity of the body and rays is 

 filled with a watery corpusculated fluid (blood); a similar 

 fluid is found in the ambulacral vessels, and probably fills all 

 the canals." 



rig. 36 represents the circulatory system in Echinus. 



