196 



HITCHCOCK'S ANATOMY 



ually shortening, so as at last to bring the prey close to the 

 surface of the body. The spot with which it is brought in 

 contact, then slowly retracts, and forms at first a shallow de- 

 pression, gradually becoming deeper and deeper, into which 

 the prey sinks little by little, for some time, however, con- 

 tinuing to project from the surface. The depression at last 

 assumes a flask-like form, by the drawing in of its margin, 

 and finally its edges close together, and its prey is entirely 

 shut in. This gradually passes to the center of the body, 

 where its soluble parts are dissolved, whilst in the mean time 

 its external portion recovers its pristine condition." 



357. Other Radiates have an alimentary canal more com- 

 plicated, having a stomach and caecal appendages that appear 

 to perform the office of a liver. Some of the Echinoderms 

 have teeth; in the Echinidse is a curious apparatus in the 

 mouth called Aristotle's lantern. 



358. Another form of the digestive apparatus consists of a 

 central cavity, w T ith branches extending through every part 

 of the body. Fig. 200 shows this in one of the Articulated 

 animals, an Arachnoid Crustacean, the Ammothea pycnog- 

 onoides. This arrangement is found in several other classes 

 of the Invertebrates. 



359. The Crustaceans are generally furnished with two 

 upper jaws, called Mandibles, which move laterally ; and be- 

 hind these, two pairs of weaker and softer lower jaws, which 

 are sometimes changed into suckers and legs. The higher 

 branches are always provided with prehensile organs for seiz- 

 ing the food, which are arranged in pairs. 



360. These are best seen in the Lobster, where they are 

 enormously developed, projecting in front of the eyes a dis- 

 tance nearly equal to that of the length of the whole body, 

 and each extremity is furnished with a powerful pair of pincers. 

 Some of the higher Crustaceans have a sort of horny teeth 

 implanted in the coats of the stomach which are worked by a 



359. What are the jaws of crustaceans ? 



