//. ACEPHALA. 



361 



into two long conjoined tubes (fig. 349, A}, which for their retrac- 

 tion need special muscles, which are attached to the valves and 

 thus cause the pallial sinus referred to above (fig. 348). 



In the shell three layers may be distinguished (fig. 350) : on the outside 

 a thin organic cuticula and below two layers largely of calcic carbonate. 

 In many these two layers are distinguished as the prismatic layer and the 

 nacreous layer, the first consisting of closely packed prisms; the nacreous 

 layer of thin lamellae generally parallel to the surface. These by their free 

 edges produce diffraction spectra and so the iridescent appearance of the 

 shell; the finer the lines thus formed the more beautiful the play of colors. 

 This is especially noticeable in the mother-of-pearl shells Meleagrina and 

 Margaritina margaritifera. When foreign substances get between mantle 

 and shell they stimulate a greater secretion of nacreous substance and 

 become surrounded by layers of it. In this way pearls are formed. 



K? K 3 



FIG. 351. Anatomy of Anodonta, the mantle, gill, and liver of the right side 

 removed, the pericardium opened. 1, 2, anterior and posterior adductors; I, II, 

 III, cerebral, pedal, and visceral ganglia; a, anus; 7. 1 , ib 2 , upper and lower limbs 

 of organs of Bojanus ; frr, branchial siphon; d, intestine; e, nephridial opening; 

 fu, foot ; 0, gonad ; Ti 1 , ft 2 , ventricle and auricle of heart ; 7c', insertion of both 

 lamellae of right gill ; fc 3 , 7c 4 , inner and outer lamellae of left gill; Z, left liver ; I', 

 its opening in ?n, stomach; rn7, pallial line; r 1 , anterior, r a , posterior retractor 

 muscle: sp, nephrostome : v, labial palpus. The arrows show the planes of sec- 

 tion of fig. 352. 



The gills lie between the mantle and the body and from their 

 lamellar character have given rise to the name Lamellibranchiata. 

 (figs. 351, 352). Two gill-leaves occur on either side, although 

 occasionally the outer or both may degenerate. Frequently the 

 gills of the two sides unite behind the body and produce a parti- 

 tion which separates the mantle cavity into a small dorsal cloacal 



