ANNULOIDA : ECHINODEKMATA. 



127 



The digestive system of the 'Ecliinus consists of a mouth, 

 armed with five long, calcareous, rod-like teeth, which per- 

 forate five triangular pyramids, the whole forming a singular 

 structure, known as 'Aristotle's lantern.' 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, 

 sub-central, or altogether excentric in position. The anus 



i. 



Pig. 33. Morphology of Echinoidea. 1. Ecliinid larva, a. Mouth ; 6. Stomach ; 

 c. Intestine ; 5. Skeleton. 



2. Diagram of Echinus. The spines and the ambulacra are represented 

 over a small portion of the test ; the vascular system is cross-shaded ; the 

 nervous system is represented by the black line. a. Anus ; b. Stomach ; c. 

 Mouth ; d and /. Vascular rings round the alimentary canal ; e. Heart ; g. 

 Test ; h. Nervous ring round the gullet ; i. Ambulacral ring or ' circular 

 canal ' round the gullet ; k k, Polian vesicles ; /. Sand canal ; m m. 

 Radiating ambulacral canal ; n. Secondary ambulacra! vesicles ; o. 

 Ambulacra, or ' tube-feet; ' p. Spines ; r. Madreporiform tubercle. 



varies considerably in its position, being usually situated 

 within the apical disc, and surrounded by the genital and 

 ocular plates. Sometimes, however, the anal aperture is 

 without the apical disc, and is removed to some distance from 

 the genital plates. The convolutions of the alimentary canal 

 are attached to the interior of the test by a delicate mesentery; 

 the surface of which, as well as that of the lining- membrane 

 of the shell, is richly ciliated, and subserves the purposes of 

 respiration. 



The proper blood- vascular system (Jig. 33, 2) consists of a 

 central, fusiform, contractile vesicle, or heart. This gives off 

 one vessel, which forms a ring round the intestine near the 

 anus, and another which passes downwards, and forms a circle 

 round the gullet, above the ' circular canal ' of the ambulacral 



