204 



LECTURE X. 



Alim. canal of Echinus. 



The digestive apparatus of the Echinus {Jig. 98.) consists of a 



mouth armed with teeth, surrounded 

 by a muscular labial membrane, and 

 five pairs of pinnate tubular ten- 

 tacula, of an oesophagus and sto- 

 mach, and of an intestine suspended 

 by a mesentery to the interior of 

 the shell, and which, after perform- 

 ing one or two circumgyrations, 

 terminates by a distinct outlet op- 

 posite to the mouth. The outer 

 margin of the lip is fringed by a 

 circle of the ophicephalous pedi- 

 cellarise, visible to the naked eye. 



The teeth (a) are five in number ; 

 tliey are calcareous, three sided 

 prisms, dense at the working apex, 

 {t,, fig. 99.), with the inner edge 

 sharp and fit for cutting, and becoming softer at the root, ^, which 

 rises above the base of the socket, is bent inwards and downwards, 

 and is inclosed in a membrane. Each tooth is implanted in a larger 

 triangular pyramid {h, and A, fig. 99.), two sides of which are 

 in close apposition with opposite sides of the adjoining pyramids, 

 and are transversely grooved like a file (A, fig. 99.), so as to 

 operate upon the alimentary matters which have been divided by 

 the incisor plates, and which are thus minutely comminuted before 

 they pass into the membranous oesophagus. The adjacent sockets 

 are connected together by short muscular fibres (/>, fig. 99.). Ten 

 additional pieces contribute to form this apparatus, which has been 

 called " Aristotle's lantern." Five of these {i, fig. 99.) are placed 



horizontally on the upper surface 

 of the lantern, occupying the basal 

 intervals of the pyramids h ; the 

 other five pieces, k, are placed 

 directly over the first, and are 

 longer and more slender, and bent 

 with the convexity upwards. 



The secretion of some simple 

 salivary follicles assists in com- 

 pleting the mastication of the 

 food. These singular representa- 

 tives of molar and incisor teeth 

 are moved upon eacli other ; and the entire pyramidal mass can be 



A. Two sockets with teeth ; B. Single socket viewed 

 on the outside. 



Echinus esculeniut. 



