XX THE ANIMAL KINGDOM. 



food is taken into the alimentary canal is called the mouth, whether reference is made 

 to tlie 'mouth' of a hydra or of a vertebrate. Although the structure of the edges 

 may differ i-adically, still in most Metazoa tlie mouth is due to an inpushing of the ec- 

 toderm, however differently the edge may be supported and elaborated. The edges 

 of the mouth are usually called the li]is, but true lijjs for the first time appear in the 

 Mammalia. The trituration or mastication of the food is accomplished among tlie in- 

 vertebrates in a variety of ways, and by organs not always truly homologous. 



The object of digestion is to reduce the food into a convenient condition, and to 

 dissolve or to transform it into tissue-food. How the products of digestion are carried 

 about in the body, so as to supply the tissues of the various organs, by the circulatory 

 organs, will be pointed out in the next section. The simplest form of the digestive 

 process may be seen by a skilled observer in the cells lining the digestive pockets or 

 chambers in the interior of a sponge or jelly-fish. Each cell has a certain amount of 

 individuality, taking in, through transitory openings in their walls, particles of food, and 

 rejecting the waste portions, much as individual Infusoria, which have no stomachs, in- 

 gest their food, and reject tlie particles which are indigestible or not needed. Not only 

 has intra-cellular digestion, as it is called, been observed in sponges and jellj'-fishes, as 

 well as in ctenophores, but also in low worms (Turbellaria). 



We will now look at the leading steps in the evolution and specialization of the di- 

 gestive cavity of animals. In the polyps, such as Hydra and Coryne, it is simply a 

 hollow in the body. The Hydra draws some little creatui'e with its tentacles into its 

 storaacli ; there it is acted upon by the juices secreted by the walls of the stomach, and 

 the hard parts rejected from the mouth. For the technical name of the digestive tract 

 as a whole, 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 partially digested and mixed witli sea-water, thus 

 forming a rude sort of blood, supplies the tissues with nourishment. In the sea-anem- 

 ones 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 echinoderms 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 ccelom), and opening externally for the rejection of waste 

 matter. 



In the worms the digestive canal becomes separated into a mouth, an oesophagus, 

 with salivary glands opening into the mouth, and there is a division of the diges- 

 tive tract into three regions — i.e., fore (oesophagus), middle (chyle-stomach), and 

 hind enteron (intestine). In the molluscs and higher worms there is a well-marked 

 sac-like stomach and an intestine, with a liver, present in certain worms and in the ascid- 

 ians and molluscs, opening into the beginning of the intestine. All these divisions of 

 the digestive tract exist still more clearly in the Crustacea and most insects. In the 

 latter, six or more excretory tubes (Malpigian vessels) discharge their contents into 

 the intestines, and in the ' respiratory tree ' of the Holothurian, and the segmental 

 organs of certain woiTns we have organs with probably similar excretory uses. 



In the vertebrates, from the lancelet to man, the alimentary canal has, ^\'ithout 

 exception, the three divisions of oesophagus, stomach, and intestine, with a liver. In 

 this branch the lungs are modified parts of originally sac-like dilatations of the first divi- 

 sion of the digestive tract. The intestine is also sub-divided in the mammals into the 

 small and large intestine and rectum, a coecum being situated at the limits between the 



