INTRODUCTION. xxi 



small and large intestine. We thus observe a gradual advance in the degree of spe- 

 cialization of the digestive organs, corresponding to the degree of complication of the 

 animal. — (Packard's Zoology.) 



We will now look at the glands which pour their secretion into the digestive canal. 

 In the worms, salivary glands send their secretion into the throat, while in the polyps 

 (ccelenterates) and many worms, and in all insects the stomach is lined with a layer of 

 colored cells which secrete bile ; in the spiders the stomach forms a set of complicated 

 coecal appendages which secrete a tiuid like bile ; in the Crustacea, and lower molluscs, 

 there is a liver formed of little glands which open into the beginning of the intestine, 

 while in the higlier molluscs and in the vertebrates we have a true specialized liver 

 merely connected with the digestive canal by its ducts. 



Thus the original food-stuff is variously treated by animals of different grades. 

 In the searauemone or any polyp, a very imperfectly digested material is pro- 

 duced, which is taken at first hand, mixed with the sea-water, and in part churned by the 

 movements of the body, in part moved about in a more orderly and thorough manner, 

 in currents formed by the cilia lining the chambers of the body. 



In the worms, and insects, etc., the chyle or products of digestion percolates, or 

 oozes through the walls of the intestine into the body cavity, and there directly mingles 

 with the blood, and is thus carried in the circulation to every part of the body, however 

 remote or minute. In the vertebrates, however, this is not so. The chyle, a much 

 more elaborate fluid than that of the lower animals, is carried by an intricate system 

 of vessels, called lymphatics, from the intestine to the blood vessels. Thus the process 

 of digestion becomes increasingly elaborate as we ascend in the animal series, and as 

 the digestive system becomes more and more complex. 



Here again we might look at the chewing apparatus or teeth, arming the mouth, by 

 which the food is made ready for digestion. To quote from the author's text-book of 

 Zoology: — Hard bodies serving as teeth occur for the first time in the animal series in 

 the sea-urchins, wiiere a definite set of calcareous dental processes or teeth, with solid 

 supports and a complicated muscular apparatus, serves for the comminution of the food, 

 which consists of decaying animals and sea-weeds. In those echinoderms which do 

 not have a solid framework of teeth, the food consists of minute forms of life, proto- 

 zoans and higher soft-bodied animals, or the free-moving young of higher animals, 

 which are carried into the mouth in currents of water or swallowed bodily with sand 

 or mud. 



Among the worms, true organs of mastication for the first time appear in the 

 Rotatoria, where the food, such as Infusoria, etc., is crushed and is partly comminuted 

 by the well-marked horny and chitinous pieces attached to the mastax. In most other 

 low worms the mouth is unarmed. In the leeches there are three, usually in the 

 annelids two, denticulated or serrate, chitinous flattened bodies, situated in the exten- 

 sible pliarynx of these worms, and suited for seizing and cutting or crushing their 

 prey. 



In the higher molluscs, such as the snails (Cephalophora) and cuttles, besides 

 one or more broad, thin pharyngeal jaws, comparable with those mentioned as existing 

 in the worms, is the lingual ribbon, admirably adapted for sawing or slicing sea-weeds, 

 and cutting and boring into hard shells, acting somewL.vt like a lapidary's wheel; this 

 organ, however, is limited in its action, and in the cuitles, the jaws, which are like a 

 parrot's beak, do the work of tearing and biting the animals serving as food, which .are 

 seized and held in place by the suckered arms. 



