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MANUAL OF ZOOLOGY. 



urchins), they are " interrupted," being restricted to the summit 

 of the test, and usually being broad and petaloid (fig. 93, D). 



As regards the digestive system, the mouth is typically 

 situated in the centre of the base ; but it may be excentric ; 

 and in one singular living form (Leskia) it is protected by 

 valvular calcareous plates. Some forms have the mouth tooth- 

 less, but others possess a complicated masticating apparatus. 

 In Echinus this consists of five long, calcareous, rod-like teeth, 

 which perforate five triangular pyramids, the whole forming a 

 singular structure, known as " Aristotle's Lantern " (fig. 96, C). 



Fig. 96. A, The masticatory apparatus of an Echinoid (Toxopneustes lividns), viewed 

 from above, with part of the alimentary canal attached to it : a (Esophagus ; b 

 Heart, with the sand-canal (c) in a groove on one side ; */The summit of the masti- 

 catory apparatus, with some of the muscles (e) of the same. B, Minute structure 

 of one of the plates of the test of an Echinus (greatly magnified), showing the calci- 

 fied areolar tissue. C, The masticatory apparatus of Sphcerechinus esculentus, 

 viewed from the inside and laterally, as seen in place : ff Peristomial margin of the 

 corona ; gg Two of the radiating ambulacral vessels, with their rows of ampullae. 



The mouth conducts by a pharynx and a tortuous oesophagus 

 to a stomach, opening into a convoluted intestine, which winds 

 round the interior of the shell, and terminates in a distinct 

 anus. The mouth is always situated at the base of the test, 

 and may be central, subcentral, or altogether excentric in 

 position. The anus varies considerably in its position, being 

 usually situated within the apical disc, and surrounded by 

 the genital and ocular plates, when the test is said to be 



