358 MOLLUSC A. 



Sub Class II. Solenogastres (Aplacophora). 



Worm-like forms without shell; the 

 foot rudimentary and at the bottom of 

 a ventral groove. The radula is also 

 reduced; in Chcetoderma it bears but a 

 single tooth. The gills are either small 

 or wanting. The usually hermaphrodite 

 animals have the gonads emptying into 

 an unpaired chamber (pericardium?) and 

 FIG. m.-Neomenia corinata, thence to the exterior by the paired 

 BSS^IS^^SS nephridia. Clmtoderma in New Eng- 

 terior;c, ventral groove. land; ^eomenia, Dondersia. 



Class II. Acephala (Lamellibranchiata, Pelecypoda). 



These have, among the molluscs, the least powers of locomo- 

 tion. Some are fixed, the majority burrow slowly through sand 

 or mud ; only a few spring by means of the foot or swim by open- 

 ing or closing the shells. Hence it is that they need more pro- 

 tection than other species, and this is afforded by the strong shells 

 in which the body can usually be completely enclosed. This shell 

 recalls that of the brachiopod in that it consists of two halves or 

 valves, but these valves are right and left rather than dorsal and 

 ventral, and hence are usually symmetrical in shape. Only when 

 the animal rests permanently on the right or left side is this sym- 

 metry lost, and then the symmetry of the soft parts is affected. 



The two lobes of the mantle which secrete the shell on their 

 outer surface arise from the back of the animal and grow down- 

 wards, forwards, and backwards, so that they envelop the whole 

 -(fig. 352). Hence the oldest and the most thickened part of the 

 .shell, the umbo, occurs near the back (fig, 347). Around this 

 the lines of growth are arranged concentrically, lines which show 

 how, by gradual growth of the mantle, the shell has increased in 

 .size. On the back the valves approach each other, and in the 

 majority are movably connected by a hinge, which consists of 

 projections (' teeth ') in one valve fitting into depressions in the 

 other. In the Brachiopoda the valves are opened by appropriate 

 muscles; in the Acephala by an elastic hinge ligament usually placed 

 dorsal to and behind the hinge. The shell is closed by adductor 

 muscles which extend through the body from shell to shell, leav- 

 ing their impressions or scars on the inner surface (fig. 347). 



