634 ZOOLOGY. 
digestive tract we may adopt Haeckel’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 Hchi- 
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 celom), and opening externally 
for the rejection of waste matter. 
In the worms the digestive canal now becomes separated 
into a mouth, an esophagus, with salivary glands opening 
into the mouth, and there is a division of the digestive tract. 
into three regions—i.e., fure (esophagus), 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 aiimen- 
tary canal has, without exception, the three divisions of cs- 
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, 2 cecum 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. 
