G34 ZOOLOGY. 



digestive tract we may iidopt Haeckcl's term enteron. In 

 the jelly-fishes the stomach opens into four or more water- 

 vascular canals or passages, by which the food, when par- 

 tially digested and mixed with sea-water, thus forming a 

 rude sort of blood, supplies the tissues with nourishment. 

 In the sea-anemones and coral polyps, the digestive cavity is 

 still more specialized, and its walls are partly separated from 

 the walls of the body, though at the posterior end the 

 stomach opens directly into the body-cavity. In the Echi- 

 noderms and worms do we find for the first time a genuine 

 digestive tube, lying in the perivisceral space (which, with 

 Haeckel, we may call the ccelum), and opening externally 

 for the rejection of waste matter. 



In the worms the digestive canal now becomes separated 

 into a mouth, an oesophagus, with salivary glands opening 

 into the mouth, and there is a division of the digestive tract 

 into three regions — i.e., fore (oesophagus), middle (chyle- 

 stomach), and hind (intestine) enteron. In the mollusks 

 and higher worms there is a well-marked sac-like stomach 

 and an intestine, with a liver, present in certain worms (in 

 the ascidians and mollusks), opening into the beginning of 

 the intestine. All these divisions of the digestive tract ex- 

 ist still more clearly in the Crustacea and most insects. In 

 the latter, six or more excretory tubes (Malpighian vessels) 

 discharge their contents into the intestines, and in the " res- 

 piratory tree " of the Holothurian and the excretory vessels 

 of certain worms we have organs with probably similar uses. 



In the vertebrates, from the lancelet to man, the alimen- 

 tary canal has, without excejjtion, the three divisions of oes- 

 ophagus, stomach, and intestine, with a liver. In this branch 

 the lungs are either, as in the lancelet, modified parts of 

 the first division of the digestive tract or originally sac-like 

 dilatations of the digestive tract. The intestine is also 

 subdivided in the mammals into the small and large intestine 

 and rectum, a coecum being situated at the limits between 

 the small and large intestine. We thus observe a gradual 

 advance in the degree of specialization of the digestive or- 

 gans corresponding to the degree of complication of the an- 

 imal. 



