102 



INVERTEBRATE ANIIIALS. 



The digestive system in Echinus consists of a mouth armed 

 ■with a curious apparatus of calcareous teeth, which opens into a 

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

 teeth form u, singular pyramidal structure known as "Aristotle's 

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

 and convoluted intestine is attached to the interior 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 fills the general cavity of the body, and 



Fig. 63. — Cickcris papillata. (After Gosse.) 



which corresponds 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 sand-canal, and which have 

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

 gangliated cord placed round the gullet, and sending five radiating 

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

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

 sacs placed in a radiating manner in the interambulacral areas, and 

 opening at the genital plates. 



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

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

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

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

 action. 



