270 COLLEGE ZOOLOGY 



Gastropoda and certain Pelecypoda). There is usually a 

 ventral muscular foot, a mantle fold, a radula, and a coelom. 

 The shell, if present, is usually univalve, bivalve, eight-parted, 

 or pen-shaped. 



The bodies of mollusks are soft (Lat. mollis = soft) and gen- 

 erally covered by a slimy integument. They are therefore 

 fitted for life in the water or in moist places. In most cases the 

 body is supported and protected by a shell. As shown in Figure 

 172, the/00/ is present in all mollusks, but is variously modified; 

 it enables the mussel to plow its way through the sand, the snail 

 to glide along, and the squid to swim through the water and cap- 

 ture its prey. The mantle is a fold of the body- wall which secretes 

 the shell. If there are two lobes, a bivalve shell is produced, as 

 in the mussel. If only one lobe is present, a univalve shell 

 is formed, as in snails. The shape of the animal does not 

 depend upon the shell so much as upon the mantle which 

 secretes it. 



The Mollusca possess a distinct cozlom which is usually 

 recognizable in the adult as (1) the pericardial cavity, and (2) the 

 cavities of the reproductive organs. 



Metabolism. — Mollusks eat both vegetable and animal food. 

 Jaws are present in many of them, especially the gastropods and 

 cephalopods. A rasping organ, the radula (Fig. 182), exists in 

 the buccal cavity of many mollusks; it consists of rows of chi- 

 tinous teeth which tear up the food by being drawn across it. In 

 the stomach the food is acted upon by secretions from the liver, 

 which is physiologically a hepato-pancreas, and may also excrete 

 waste products into the alimentary canal. 



The cavities which contain the blood represent the hcemoccel. 

 The blood is forced through these cavities by the muscular con- 

 tractions of the heart. Oxygen, absorbed food, and excretory sub- 

 stances are transported by it. Respiration takes place either in 

 the gills or in the mantle. Most of the fresh-water and land- 

 snails (pulmonate gastropods) take air into the mantle cavity, 

 which thus serves the purpose of a lung. The Pelecypoda, 



