3i = 



MANUAL OF ELEMENTARY ZOOLOGY 



of the young mussel. On the inside of the shell may be 

 seen the marks of attachment of the adductor, retractor, 

 and protractor muscles presently to be mentioned, and 

 parallel with its edge is a mark known as the pallial line, 

 where the fold of the body-wall known as the mantle is 

 attached. Above the hinge the two valves are joined by an 

 elastic ligament, which pulls them together and thus causes 

 them to gape below when the adductor muscles are relaxed. 

 To open the shell of a living mussel the blade of a knife is 

 passed between the valves and they are prised apart till the 



muscles can be cut close 

 to the shell on one side. 

 The body of the animal is 

 then found to be soft, with- 

 out a cuticle, and provided 

 with a flap of tissue which 

 hangs down on each side 

 and covers the other organs. 

 This is the mantle. It has 

 a thick edge which secretes 

 the two outer layers of the 

 shell, while the pearly layer 

 is laid down by the whole 

 outer surface of the mantle 

 and skin of the back. 

 Pearls are formed in the 

 same way in pockets ol 

 the mantle surface around 

 foreign bodies which have 

 intruded between mantle and shell. The origin o\ the 

 mantle from the side of the body is not straight but 

 higher in the middle than near the two ends, though at the 

 extreme ends it turns upwards to the hinge line. At the 

 hind end each mantle edge is fused for some distance with 

 its fellow ; it then separates widely from it twice, so as to 

 form the figure of 8 already mentioned, and lies against 

 its fellow for the rest of its length. The upper opening is 

 known as the dorsal siphon, the lower as the ventral siphon. 

 The lips of the latter bear a fringe of small tentacles. 

 The space enclosed by the two mantle lobes is known as 

 the mantle cavity. 



■ i. g . 



-ii g . 



Fig. 221. — Part of the shell of a 

 swan mussel, seen from above. 



I.g., Lines of growth ; lig., ligament ; 

 lib. , umbo. 



