II. ACEPHALA 



31 !> 



excavated at the posterior end, so that when brought together two open- 

 ings, an upper and a lower, result (fig. 320, C). The lower of these is 

 the branchial opening by which fresh water passes into the mantle cham- 

 ber; it flows out after passing over the gills, along with the faeces, through 

 the upper or cloacal opening. In many bivalves the free edges of the 

 mantle grow together, leaving three openings (fig. 320, B), one for the 

 protrusion of the foot, the others the two just described, now called the 

 branchial and cloacal sip/ions. By further development the margins of 

 these openings are drawn out into two long conjoined siphonal tubes (A), 

 which for their retraction need special muscles; these are attached to the 

 valves and thus cause the pallial sinus referred to above. 



In the shell there are three layers (fig. 321): 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 laver, the first 

 consisting of closely packed prisms; the nacreous layer of thin lamellae generally 



.-..- 



FIG. 321. Section of shell of Anodonta. c, cuticula; p, prismatic layer; /, nacreous 



layer. 



parallel to the surface. These 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 shell J/Y/<u- 

 grina and many Unionidae. When foreign substances get between mantle and 

 shell they stimulate a greater secretion of nacreous substance and become 

 surrounded by layers of it. Pearls are formed in this way. 



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

 character have given rise to the name Lamellibranchiata (figs. 322, 323). 

 Two gill-leaves occur on either side, although occasionally the outer or 

 both may degenerate. Frequently the inner gills of the two sides unite 

 behind the body and produce a partition which divides the mantle cavity 

 into a small dorsal cloaca and the larger lower branchial chamber. The 

 anus and the water which has passed the gills empty into the cloaca 

 which connects with the excurrent siphon. The incurrent siphon leads 



