416 



MOLLUSCS. 



more often hide in crevices or at the roots of seaweed, mooring themselves by a 

 byssus. The shells are very irregular, their form varying according to the hole or 



crevice they inhabit. In the Gastrochcenidce, 

 forming the last family of this group, Gastrocltaina 

 comprises bivalves which live buried in the sand. 

 These form a long, slender, club-shaped fragile 

 tube, covered with adhering particles of sand, and 

 divided off by a partition into two portions, the 

 anterior containing the shell, the posterior or 

 narrower end the siphons. The animal of the 

 allied Rocellaria is similar, but forms no tube, and 

 has the habit of boring into solid rock, shells, and 

 other substances; R. dab la being found in lime- 

 stone, and even granite, on the British coasts. 



animal (a) and case (b) of Rocellaria. 



Suborder Pholadacea. 



The boring Pholadidcu and Tcredinidce are the only families contained in this 

 suborder of the group, the former perforating clay, chalk, limestone, and even 

 gneiss. Their shells are always white, thin, but hard and strong, and ornamented 

 with prickly rasp-like sculpture. They gape all round the valves, meeting only at 

 the hinge and the opposite margin. Accessory plates generally occupy the vacant 

 spaces. The valves 

 have no hinge-teeth, 

 and are not connected 

 by a well - defined 

 ligament, like most 

 bivalves. The 



animals have long 

 united siphons, 

 fringed at the aper- 

 tures, and enclosed in 

 a tough skin, which 

 is often protected by 

 cartilaginous cup-like 

 processes. In the 

 typical PI tolas the 

 foot is well devel- 

 oped, and probably 

 forms the principal 

 excavating instru- 

 ment ; the shell being 

 used as a file to 



enlarge the crypt as the creature grows. Xylophaga and Martesia bore into 

 floating wood. Species of this family are met with everywhere, and about half a 

 dozen occur on the British coasts. In some parts of EurojDe Pholas is considered 



Pholas in its burrow (nat. size). 



