634 ZOOLOGT. 



digestiye tract ire may adopt Haeckel's term enferon. la 

 the jelly-fishes the stomach opens into four or m.ore -water- 

 Tasculcr 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 

 stiU 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 genuin& 

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

 Haeckel, we may call the cml&m), 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 ijiouth, 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. AU 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 excretorv vessels- 

 of certain worms we have organs with probablv similar uses. 



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

 tary canal has. without exception, tlie three divisions of ces- 

 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. 



