2l8 ELEMENTARY ZOOLOGY. 



uses it is put to; respiration, how accomplished; locomo- 

 tion, organs of, and their efficiency; the amount of food 

 in the egg and the manner of development. 



One topic might be assigned to each student, if the 

 time is short, and a special report be brought on each 

 topic before the whole class. 



239. The internal organization of the mollusks is the most complex 

 we have encountered in the branches we have studied. All the 

 important functions have specialized organs, and some of them are 

 highly specialized. The circulatory system, for example, is superior 

 even to that of the next branch the arthropods, to which the cray- 

 fish belongs. The respiration may take place through the wall of 

 the mantle, or by means of gills (water-breathers), or by a sac (air- 

 breathers). These structures, while in the mantle chamber, are not 

 in the body proper. Within the body are digestive organs, special 

 kidneys or excretory organs, reproductive organs which are often 

 very complex, and the nervous system. The body cavity is not so 

 large as in the segmented worms. 



240. The Digestive Organs. These differ greatly in accordance 

 with the food used and the general structure of the animal. There 

 is both a mouth and an anal opening, and the digestive tube usually 

 runs the length of the body. In some forms, especially those in 

 spiral shells, the tube passes up the spiral, turns, and comes back, 

 opening close to the mouth. The snails have a roughened ribbon- 

 like tongue by means of which they rasp off the substance of the 

 leaves and other objects on which they feed. The clams have only 

 a hole in the wall, to which cilia waft the small particles of food 

 carried by the waters. There are no teeth, no tongue, no tentacles 

 for holding food. The cephalopods have powerful beak-like jaws, 

 suited to their carnivorous habit. There are always specialized 

 stomach and intestine, and usually a crop in front of the stomach. 

 The principal glands of digestion are salivary glands opening near 

 the mouth, and the liver, which is often very large. 



241. Circulatory Organs. The heart in the mollusks has a muscu- 

 lar ventricle which pumps the blood into the arteries. The heart 

 handles pure blood after it has passed through the gills. Auricles 

 (varying from one to four) receive the blood from the veins and 

 return it to the ventricle. The blood passes by way of the arteries 

 to the capillaries of the system in the various tissues of the body; 



