148 



PRACTICAL ZOOLOGY 



growth, since there may be more than one growth period during 

 the year. 



Structure of the Shell. If we break a part of the shell, we 

 find that the inner surface produces an iridescent sheen in the 

 light; this is the nacreous layer or mother-of-pearl. Between 

 this layer and the outside is a stratum of calcium carbonate 



crystals, the prismatic layer; and on 

 the outer surface is a thin, horny layer, 

 the periostracum, which protects the 

 other layers from being dissolved 

 away by the carbonic acid in the 

 water. 



Movement of the Valves of the 

 Shell. The two valves of the shell 

 are held together at the upper, dorsal 

 edges by an elastic, ligamentous hinge 

 (Fig. 85, i o) and in some species fit 

 together by means of toothlike pro- 

 jections. The elasticity of the hinge 

 tends to force the valves open, but 

 they are held closed or allowed to 

 spring open, to any desired extent by 

 a pair of strong bands of muscles, the 

 adductors, which extend across from 

 one valve to the other, one near the 

 anterior, the other near the posterior 

 edge of the shell (Fig. 84, aam, pam). 

 When an animal dies, the adductor 

 muscles relax and the valves open. This is why the shells of 

 dead mussels are always open. 



Water Current in the Mussel. * When closed or nearly closed, 

 there is within the shell a rather large cavity in which the body 

 lies; this is the mantle cavity (Fig. 85). The mantle cavity 

 communicates with the water surrounding the mussel by means 

 of two tubes or siphons, one above the other, formed by the 



FIG. 85. Cross section of 

 a mussel. 



1, right auricle; 2, epi- 

 branchial chamber; 3, ven- 

 tricle ; 4, vena cava ; 5, non- 

 glandular part of kidney ; 6, 

 glandular part of kidney; 

 7, intestine in foot; 8, peri- 

 cardium; 9, shell; 10, liga- 

 ment of shell. (After 

 Howes.) 



