NUTRITION IN THE ASTERIAS. 



91 



animal. We here find an oesophagus, or narrow 

 tube, leading from the mouth to the stomach ; and 

 the stomach is continued into a regular intestine, 

 which takes two turns in the cavity of the body, 

 before it terminates. In these animals, as in the 

 former, ciliary motions take place over nearly the 

 whole surface of the internal cavities. 



The alimentary tube in the lower animals fre- 

 quently exhibits dilatations in different parts: these, 

 if situated in the beginning of the canal, may be 

 considered as a succession of stomachs ; while those 

 that occur in the advanced portions are more pro- 

 perly denominated the great intestine, by way of 

 distinction from the middle portions of the tube, 

 which are generally narrower, and are termed the 

 small intestine. We often see blind pouches, or 

 cceca, projecting from different parts of the canal ; 

 this is the case with the intestine of the Aphroclita 

 aculeata, or sea-mouse. The intestine, being gene- 

 rally longer than the body, is obliged to be folded 

 many times within the cavity it occupies, and to 

 take a winding course. In some cases, on the 

 other hand, the alimentary tube passes in nearly a 

 straight line through the body, with scarcely any 

 variation in its diameter : this is the case with the 

 Ascaris, which is a long cylindric worm ; and 

 nearly so with the Lumhricus terrestris, or earth- 

 worm. In the Nais, on the 

 contrary, as is shown in Fig. 

 259, the alimentary tube pre- 

 sents a series of dilatations, 

 which, from the transparency 

 of the skin, may be easily 

 seen in the living animal. The food taken in by 



