PELECYPODA. 
163 
sliglitly attached to the distal tubercle, and is strengthened by four 
projecting and slightly twisted longitudinal blades, placed at right 
angles to each other and slightly thickened and rounded at the 
outer edges, which cause the dart 
to revolve slightly during its pro- 
trusion. The blades gTadually 
diminish towards the point of the 
Fig. 329 -Dart of Heiu aspersa X i, ^^^rt aiid inore abruptly basally, 
Christchurch, Hants. conuected together at 
intervals by crescentic and very thin calcareous films. The dart is 
everted, and used by the animal as an excitant to its prospective 
mate, during the amorous preludes which lead up to, and precede 
sexual congress. 
The Pelecypoda, on the other hand, differ widely from the Gastro- 
poda, as they are exclusively aquatic animals which would appear to 
liave adopted a more or less inactive and sedentary life, and retained 
more completely than the Gastropoda the original bilateral symmetry 
of their external and internal organs, but lost l)y degeneration or 
atrophy such organs as were especially adapted to an active loco- 
inotory existence, the probability of their former possession of sucli 
organs being corroborated by brief traces of their existence during 
larval life. The great development of vibratile cilia over the free 
surface, more especially of the largely developed and paired ctenidia 
or gills, render it probable that the continuous flow of food-laden 
currents directed towards the mouth by their action, would also tend 
by the easy nutrition thus secured to confirm the sedentary habits 
of the animal and contribute to the degeneration of the cephalic 
region by rendering it functionally unnecessary. The development of 
the ample mantle lobes and their encompassing and protecting shells 
becomes a necessary corollary of the adoption of inactive habits, as the 
degeneration of the locomotory organs would naturally prevent active 
escape from apprehended danger. 
The Pelecypoda are broadly divided into three gTOups, viz. : 
Isomya, Heteromya, and Monomya, characterized by the relative 
degree of development of the adductor muscles of the shell, and which 
like the, nerve cords in the Gastropoda equally indicate successive 
stages of specialization. 
