IOl 



INVEUTEBHATE ANIMALS. 



The digestive system in Echinns consists of a mouth armed 

 with a curious apparatus of calcareous teeth, wliich opens into a 

 gullet, which ill turn conducts to a long intestinal tube. The 

 teeth form a siugul.ir pyramidal structure known as "Aristotle's 

 lantern"; but many ,Sea - urchins are without teeth. The long 

 and convoluteil intestine is attached to the intei'ior of the shell 

 by a delicate membrane or " mesentery," and terminates in a dis- 

 tinct vent. The surface of the mesentery, as well as that of the 

 lining membrane of the shell, is richly ciliated, and thus serves to 

 distribute the fluid which iills tlie geiier.al cavity of the Ixidy, and 



. m. — Chluris papilUilu. (AftiT Goss.'.) 



which corresiionds with the blood, to all parts of the body. It is 

 doubtful if any true blood-system is present, but there is present 

 a .system of tubes, of which the principal portion is a long fusi- 

 form vessel running parallel with the saud-canal, and which have 

 been legarded as blood-vessels. The nervous .system consists of a 

 gangliated ccjrd jilaced round the gullet, and sending five radiating 

 branches along the anibulacral areas. The two sexes are distinct, but 

 in both the rejiroductive organs are in the form of five membranous 

 sacs placed in a ra<liating manner in the interamlmlacral areas, and 

 opening at the genital ]ilates. 



The majority of the Sea-urchins iire found at moderate depths in 

 the sea, especially in the neighbourhood of oyster-banks. Others 

 spend their existence liiiiied in the sand ; and one species excavates 

 holes for itself in the solid rock, ajiparently by some mechanical 

 action. 



