ECHIXODERMATA. 197 



caecum is suspended from the dorsal wall of the ray by a pair of 

 peritoneal folds {fig. 95, n, n), which include a space communicating 

 with the central part of. the body, at the root of the caeca : the ca^cal 

 area and appendages are shown in transverse section at g, g,fig- 95.* 



The alimentary canal is lined by a ciliated epithelium. The 

 Crinoids use the delicate tentacles of their pinnules and arms to seize 

 their prey and bring it to the mouth : the Asteroids use their 

 prehensile rays, or the ray-like prolongations of the body with the 

 suctorious feet, and also their pedicellariae, for the same purpose. As 

 the star-fishes feed on decaying portions of animal substance, they may 

 be gifted with the sense of smell. The lip in Comatula is simple ; in 

 Asterias it is beset with hard papillae, which extend along the angles 

 of the mouth ; in OpJiiura the entering angles of the mouth are beset 

 with calcareous teeth, and there are soft tentacles in the intervening 

 chinks. The mouth in Ophiura is divided from the stomach by a cir- 

 cular sphincter : the sides of the stomach bulge out into, usually ten, 

 caeca, which in Asterophyton are subdivided into sacculi : but cajcal 

 appendages are not continued from the central stomach into the rays 

 of the Ophiura, which seem to be rather appendages to, than divisions 

 of, the body. In the Comatula the alimentary canal presents a higher 

 type of structure : there is a slightly convoluted intestinal canal 

 which terminates by a distinct tubular anus opening on the ventral 

 side of the disc, near the mouth. 



Professor Tiedemann, in his celebrated monograph on the EchinO' 

 derma^, has successfully demonstrated a vascular system in all the 

 leading forms of that class. In the Asteracanthioii rubens the vessels 

 which absorb the chyle from the digestive sac terminate, after a 

 series of reticulate anastomoses, in a circular trunk, which likewise 

 receives branches from the radiated caeca. The venous circle com- 

 municates by means of a dilated tube {fig. 91, h), regarded as a 

 rudimental form of heart, with an arterial circle surrounding the 

 gullet, which lies internal to, and is distinct from, the chylaqueous 

 circular tube. The arterial circle sends off branches which diverge 

 to the rays and other parts of the body. The cardiac tube ascends 

 from the depression in the madreporic plate, and accompanies the 

 sand-canal, h, which opens into the chylaqueous ring. From this 

 annular vessel five chylaqueous canals diverge, and extend along the 

 ambulacral spaces, and supply the tubular feet. I have not been able 

 to trace any direct communication between the true vascular system 

 of the Asterias and the system of chylaqueous canals, which, by their 

 connection with contractile pyriform diverticula, govern the supply of 



* CLX- p. 36. t CLIX. 



o 3 



