270 COLLEGE ZOOLOGY 



GASTROPODA and certain PELECYPODA). There is usually a 

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

 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 foot 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 ccelom which is usually 

 recognizable in the adult as (i) 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. 



t The cavities which contain the blood represent the hamoccel. 

 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, 



