504 MANUAL OF THE MOLLUSCA. 



mantle closed in front, except tlie pedal orifice ; siphons large, 

 elongated, united nearly to their ends ; orifices fringed ; gills 

 narrow, prolonged into the exhalent siphon, attached through- 

 out, closing the branchial chamber ; palpi long ; anterior shell- 

 muscle acting as substitute for a ligament. 



The Pholadidcs perforate all substances that are softer than 

 their own yalves (p. 394) ;* the burrows of Pholas are vertical, 

 quite symmetrical, and seldom in contact.' The ship-worms 

 (Teredines) also make symmetrical perforations, and however 

 tortuous and crowded never invade each other, guided either by 

 the sense of hearing or by the yielding of the wood. The burrow 

 has frequently a calcareous lining, within which the shell 

 remains free; Teredina cements its valves to this tube when 

 full-grown. The opening of the burrow, at first very minute, 

 may become enlarged progressively by the friction of the siphons, 

 which are furnished with a rough epithelium ; but it usually 

 widens with much more rapidity by the ivasting of the surface. 

 As the timber decomposes the shelly tubes of the Teredo project, 

 and as the beach wears away the pJiolas burrows deeper. 



Pholas, L. Piddock. 



Etymology, jpholas, a buiTOwing shell-fish, from pholeo, to 

 bore. 



Synonyms, Dactylina, Gray. Barnea, Eisso. 



Txjpe, P. dactylus. Fig. 269. 



Example, P. Bakeri, PI. XXIII., Fig. 19. 



Shell elongated, cylindrical ; dorsal margin protected by acces- 

 sory valves ; pallial sinus reaching the centre of the shell. 



Animal with a large truncated foot, filling the pedal opening ; 

 body with a fin-like termination ; combined siphons large, cylin- 

 drical, with fringed orifices. 



* M. Cailliaucl has proved that these valves are quite equal to the work of boring in 

 limestone, by imitating the natural conditions as nearly as possible, and making such a 

 hole with them. Mr. Kobertson also, has kept the living Pholades in blocks of chalk, by 

 the sea-side at Brighton, and has watched the progress of the work. They turn from 

 side to side, never going more than half round in their bui-row, and cease to work as 

 Boon as the hole is deep enough to shelter them ; the chalk powder is ejected at inter- 

 vals by spasmodic contractions from the branchial siphon, the space between the shell 

 and bun-ow being filled with this mud. (Journ. Conch., 1853, p. 311.) It is to be re- 

 marked that the condition of the Pholades is always related to the nature of tlio 

 material in which tliey are found burrowing ; in soft sea-beds they attain the largest 

 size and greatest perfection; whilst in hard, and especially gritty rock, they are 

 dwai-fed in size, and all prominent points and ridges appear worn by friction. No 

 notice has been taken of the hypothesis which ascribes tlie perforation of rocks, &c., 

 to ciliaiij action, because, in fact, there is no cuiTent between the shell or siphons and 

 the wall of the tube. 



