256 



L O I VER IN VER TEBRA TES. 



on the outer emls of the rods and cones. lu the siphonal eyes found in iSoleii, etc., we 

 have the merest apology for a visual organ. 



The sexes of the acephals are usually separate, though in rare instances they are 

 united in the same individual. The genital glands are on either side of the body, ami 

 eini)ty by paired ducts. The eggs are either cast free in the water or are retained for 

 a time between the lamelhe of the gills of the parent. The veliger presents a promi- 

 nent difference from that of gasteropods, in that the primitively simple shell soon 

 becomes bivalve. The peculiar larval form known as glochidium will be mentioned 

 in connection with the Unionidie farther on. 



Lastly, in our general account, comes the shell, which occupies so important a place 

 in existing schemes of classification, and with it may be mentioned some of the fea- 

 tures of anatomy which have been neglected in the preceding page. On examining 

 the outer surface of any bivalve shell one notes the lines of growth concentrically 

 arranged. These have as a centre an elevated portion of the shell known as the umbo, 

 the jjosition of which marks the dorsal border. Usually this umbo points towards one 

 end of the shell, which may thus be recognized as the anterior. Having these land- 

 marks, we can readily decide the question of right and left. At the dorsal margin, 

 where the two valves join, is the hinge line, and just in front of the umbo is fre- 

 quently a distinct area, half on each shell, the lunule. 



Now for the mechanism which opens and closes the valves. The closure is 

 effected by one or two transverse muscles (adductors they are called) which pass from 

 one shell to the other, and by their contraction the two valves are approximated. No 

 divaricators exist, but instead the valves are separated, the moment the muscles arc 

 relaxed, by means of an elastic ligament. This ligament may be either external or 

 internal. In the former case, as shown in Fig. 265 A., the ligament connects the two 

 valves, and by its contraction spreads them. In the other (Fig. 265 13.,) the internal 

 ligament is placed between two portions of the shell, so that when closed it is com- 

 pressed, but upon relaxation of the muscles the elasticity and expansion of the 

 ligament forces the valves apart. 



On the intei'nal surface of the shell we also find certain features which are made 

 prominent in systematic work. Just where the two valves join together is the hinge, 



usually provided with j)rojections 

 and depressions, forming what are 

 known as teeth. Those in the cen- 

 ti'e are known as the cardinal, and 

 those frequently present at the sides 

 as the lateral teeth. Near the 

 hinge line, but on the inner, con- 

 cave surface, will be found one or 

 two approximately o\al marks, the 

 imj)ressions produced by the attach- 

 ment of the adductor muscles. 

 When only one muscle is present, 

 it is morphologically the posterior 

 one. Usually close to these may 

 be seen other similar but smaller 

 scars, marking the spots where the muscles of the foot had their origin. Going around 

 the margin of the shell is a line, more or less distinct, called the pallial line, which 



Fig. 265. — Diagram showing the hinge ligament, internal and 

 external; /, ligament; m, muscle. 



