Class 3. Acephala. 



309 



piratory syphons^ which can be protruded from the shelly and 

 may attain a considerable length; usually they are connected 

 together^ and then appear externally like a single tube (sometimes 

 forked at one end), but this is divided internally into two by a 

 septum; they are rarely separated externally. In the forms 

 with these syphons the edges of the mantle have often undergone 

 concrescence over so large an extent that the pedal opening is 

 considerably decreased. 



The shell lies external to the mantle, but closely upon it. It 

 is divided into two halves,- connected above by a flexible band, the 

 ligament, which will be further described below. The two halves 

 of the shell are, as a rule, essentially similar; they are more or 



Fig. 256. — Eight shell valves of two different LamelHhranohs, seen from within. 

 A with simple, B with incurved pallial line, 6 ligament, I pallial line, m impressions of 

 adductor muscles, w umbo, s hinge. — Orig. 



less convex, and usually exhibit a projecting hump dorsally, the 

 so-called umbo (the oldest portion of the shell), which generally lies 

 nearer to the anterior than the posterior end. The upper edge of each 

 valve usually possesses a tooth or ledge, which fits in between 

 corresponding portions of the other half, forming a hinge (c a r d o) ; 

 in not a few this is absent, or only feebly developed. When the 

 shell is closed the edges of the valves fit closely together, so that the 

 soft parts of the animal are completely shut in : occasionally, however, 

 the shell is cleft in one or more places, posteriorly ; for instance, in 

 those which are provided with large syphons, and anteriorly in those 

 which are attached by means of a byssus (p. 811). The closure of the 

 shell is effected by adductor muscles, usually two in number, 

 which run transversely across the animal, one in the anteriot, the 



