336 



CHARACTERS OF INVERTEBRATE ANIMALS 



shaped, and the sharp beaks are placed at the extreme front end. 

 There are no siphons, but merely two apertures, as in the Fresh- 

 water Mussel, the inhalent one being very large and fringed. 

 The small dark foot, though capable of being used as a locomotor 

 organ, is not in constant employment as in free bivalves, which 

 probably accounts for its relatively small size. The byssus arises 

 from a deep pit behind the foot, and though it is commonly found 



Fig. 196. — Piddocks [Pkolas dactyhts) in their Burrows 



mooring the animal, it can be cast off if necessary, enabling the 

 animal to creep away to some more desirable spot. The anterior 

 adductor muscle is much smaller than the other, and this point 

 is of special interest, because it foreshadows cases where only the 

 posterior adductor is present in the adult. This is a tendency 

 to specialization, but the gills on the other hand are simpler in 

 structure than those of a Fresh-water Mussel, for though they 

 consist of two plates on each side, yet each of these can easily 

 be broken up into distinct filaments. The gill is, in fact, a some- 

 what modified plume-gill in which the separate side-branches of 

 the plume have not yet firmly united into plates, as in the fresh- 

 water mussel. 



The Ark-Shells constitute a family belonging to the same 



