148 MANUAL OF ZOOLOGY. 



lated, perforate plate on the proximal side of the preceding 

 group.. The centre of the suctorial disc is perforated by an 

 aperture conducting to the interior of the ambulacral tube- 

 foot." This perforation of the suctorial discs of the ambulacra, 

 though affirmed by Valentin, is denied by Miiller ; and it is 

 difficult to believe that it would not impair the functions of 

 the feet in the act of protrusion. 



The digestive system of the Echinus consists of a mouth, 

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

 forate five triangular pyramids, the whole forming a singular 

 stmcture, known as " Aristotle's Lantern." The mouth con- 

 ducts by a pharynx and a tortuous oesophagus to a stomach, 



Fig. 41. — ^Morphology of Echinoidea. i. Echinid larva, a Mouth; h Stomach; c 

 Intestine ; s Skeleton. 



3. Diagram of Echinus. The spines and the amhulacra are represented over a 

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

 is represented by tlie black line, a Anns ; b Stomach ; c Mouth ; d ^nAf Vascular 

 rings round the alimen'tary canal ; e Heart ; g Test ; h Nervous ring round the 

 gullet ; i Ambulacral ring or " circular canal" round the gullet ; kk Polian vesicles ; 

 /Sand canal ; tn m Radiating ambulacral canal ; n Secondary ambulacral vesicles; 

 o Ambulacral tubes, or " tube-feet" ; p Spines ; r Madreporiform tubercle. 



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 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 " regular." Some- 

 times, however, the anal aperture is without the apical disc, 

 and is removed to some distance from the genital plates, when 

 the test is said to be " irregular." The convolutions of the 

 ahmentary canal are attached to the interior of the test by a 

 delicate mesentery ; the surface of which, as well as that of the 



