578 



INVERTEBRATE MORPHOLOGY. 



pharynx surrounded by a complicated calcareous masticatory 

 apparatus usually termed Aristotle's lantern (Fig. 265, Al, and 

 Fig. 266). When most highly developed it has the form of a 

 pentagonal pyramid, whose apex is directed towards the 

 mouth and consists of five similar portions united together. 

 Each portion contains an elongated ribbonlike tooth (Fig. 

 266, t) lying in an interradius and projecting slightly beyond 

 the lips of the mouth, though for the greater portion of its 

 g length imbedded in a calcareous 



socket or alveolus (a) composed of 

 a right and a left half united above 

 by epiphyses (e). Between each 

 pair of alveoli, at their basal ends 

 is another calcareous piece termed 

 a radius, and below each of these, 

 i.e. on its oral surfaces, lies another 

 piece, the radula (r). Muscles pass 

 to this complicated apparatus from 

 the auriculae and from one piece to 

 the other, producing approximation 

 and divarication of the projecting 

 tips of the teeth. The presence 

 of this apparatus brings it about 

 that the circumoral hydrocoel and 

 lacunar rings are forced back some distance from the mouth', 

 surrounding the oesophagus just where it leaves the lantern. 

 It seems well accordingly to speak of these rings as being 

 perioesophageal rather than circumoral. 



On leaving the lantern the digestive tract, starting in the 

 interradius I)E, passes around the coelomic cavity in the direc- 

 tion of the hands of a watch, until it reaches the interradius 

 CD, when it bends abruptly on itself and, on another plane, 

 nearer the aboral surface, retraces its course almost to its 

 point of starting, whence it passes to the anus. The portion 

 of the intestine immediately succeeding the pharynx is termed 

 the oesophagus and is succeeded by a slightly wider intestine, 

 the junction of the two parts being in some forms further 

 indicated by the occurrence at that point of a large coecum. 

 As a rule, however, appendages to the digestive tract are rare, 



Fig. 266 —Aristotle's Lan 

 TERN PROM Arbacia. 

 a = alveolus. 

 e = epiphysis. 

 r = radula. 

 i = tooth. 



