288 



COLLEGE ZOOLOGY 



gins; a small vesicle (statocyst) containing 

 a calcareous concretion (statolith) lies a 

 short way behind the pedal ganglia; it is an 

 organ of equilibrium. A thick patch of yel- 

 low epithelial cells (osphradium) covers 

 each visceral ganglion. The osphradia are 

 thought by most writers to be useful in de- 

 tecting foreign materials in water. The 

 edges of the mantle are provided with 



sensory cells, probably sensitive to contact 

 and light. 



Reproduction 



Clams are usually either male or female; 

 a few are hermaphroditic. The reproductive 

 organs are situated in the visceral mass (Fig. 

 174). Some clams have interesting life 



Glochidium 

 (end view) 



Shell 



Adductor muscle 



Glochidium ^ 



(side view) n ^rom exhalent siphon 

 » ^ »' \J o . ^^^ 



••*.*♦ O o.o 



Glochidia discharged ■♦• yN 



jlochi( 

 n gills 



Glochidia become encysted 

 in gills and fins of fish 



Adult female clam 



Fertilized eggs develop 



into glochidia in 



..,.^ modified outer gill 



(brood pouch) 



Young clam 



Figure 177. Life cycle of a fresh-water clam. The larval clam or glochidium passes out of 

 the excurrent siphon of a female, attaches itself to the gills of a fish; finally, the glochidium 

 drops from the fish host to take up a free-living existence as a young clam. 



histories in which the young stage is para- 

 sitic on fish (Fig. 177). The eggs develop 

 into a peculiar larva known as a glochidium, 

 a modified veliger (Fig. 177). In Anodonta 

 the eggs are usually fertilized in August, 

 and the glochidia which develop from them 

 remain in the gills of the mother all winter. 

 In the following spring they are discharged; 

 and, if they chance to come in contact with 

 the external parts of a fish, this contact 

 stimulus causes them to seize hold of it bv 

 closing the valves of their shells. The skin 

 of the fish grows around them, forming 

 "worms" or "blackheads." After a parasitic 

 life within the tissues of the fish, from three 

 to many weeks, the young clam is liberated 

 and takes up a free existence. As a result of 

 this parasitic habit, clams are widely dis- 

 persed by the migrations of the fish. 



OTHER MOLLUSKS 



General characteristics 

 of moliusks 



The bodies of moliusks are soft and gen- 

 erally covered by a moist integument. They 

 are therefore fitted for life in water or in 

 moist places. 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. A 

 modified coelom is usually recognizable in 

 the adult as the pericardial cavity and the 

 cavities of the reproductive organs. 



Moliusks eat both vegetable and animal 

 food. Jaws are present in most gastropods 

 and all cephalopods. A rasping organ, the 



