260 



ZOOLOGY 



M 



Vertical section of shell and mantle of a mollusk; 

 C, periostracum; P, prismatic layer; L, laminated 

 layer; S, shell; M, mantle. (After Claus.) 



up of material much like that 

 which forms a cow's horn, or 

 chitin ; a middle layer composed 

 of tiny prisms of lime held in 

 by the horny material (this 

 layer is called prismatic layer) ; 

 and an inner layer (the lami- 

 nated layer), made up of layers 

 of lime and horn alternating 

 parallel to the surface of the 

 shell. The inner layer is formed 

 by the action of the whole sur- 

 face of the mantle. The two 

 outer layers are made by the 

 edge of the mantle only. So a 

 shell grows in thickness largely 

 from the inner surface of the 

 mantle, while it grows in diam- 

 eter from the edge of the mantle 

 only. 



The Open Shell. — 'PuU the 

 shells completely open. Find 

 on the dorsal side projections 

 and grooves which fit into each 

 other when the shell is closed. These are the hinge teeth. Compare the 

 number in each shell. How might they be of use to the animal? Find 

 the marks on the shell where the adductor muscles were fastened. What 

 was the use of the adductor muscles ? Why do dead mussels always have 

 the shell partly open ? 



Draw one opened valve showing all above parts. Label the anterior and 

 posterior adductor muscle scars, according to position. 



Body and Mantle Cavity. — In one valve lies the body of the clam. If we 

 remove the mantle, we shall find under a roundish soft mass, the body, or 

 visceral mass. Surrounding the visceral mass but ventral to it is a cav- 

 ity bounded on the outside by the inner surface of the mantle. This is 

 the mantle cavity. In life this cavity is full 

 of water. See if you can discover how and 

 where water gets in. In a living mussel the 

 posterior edge of the mantle on the right 

 side is folded so as to fit with the adjoining 

 edge of the mantle on the left side. The 

 funnel-like openings thus formed are called 

 siphons. 



Siphons. — The siphons can best be seen 

 in living mussels which have been left quiet 

 for some time in an open trough or tank. If 

 a little powdered carmine is allowed to drop 

 from a medicine dropper close to the siphons 

 (the fringed edges of which may be seen ex- 

 tending from the shell), a current of water 

 will be seen to draw in and expel the car- 

 mine grains. Where is the incurrent siphon 

 with reference to the excurrent? (In the Cross section of a mollusk; .4. mantle 

 "long-necked" or "soft" clam the siphons cavity; o, shell; 6, gills; 5, cloacal 

 are greatly developed and are made of cavity; k, body. 



