504 MANUAL OF THE MOLLUSCA. 
mantle closed in front, except the 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 Pholadide perforate all substances that are softer than 
their own valves (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 inyade each other, guided either by 
the sense of hearing or by the yielding of the wood. The burrow 
has frequently a calcareous liming, 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 wasting of the surface. 
As the timber decomposes the shelly tubes of the Teredo project, 
and as the beach wears away the pholas burrows deeper. 
PHOLAS, L. Piddock. i 
Etymology, pholas, a burrowing sheli-fish, from pholeo, to 
bore. 
Synonyms, Dactylina, Gray. Barnea, Risso. 
Type, P. dactylus, Fig. 269. 
Example, P. Bakeri, Pl. XXTII., 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. Cailliaud 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. Robertson also, has kept the living Fholades 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 burrow, and cease to work as 
soon 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 burrow 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 the 
material in which they are found burrowing; in soft sea-beds they attain the largest 
size and greatest perfection; whilst in hard, and especially gritty rock, they are 
dwarfed in size, and all prominent points and ridges appear worn by friction. No 
notice has been taken of the hypothesis which ascribes the perforation of rocks, &c., 
to ciliary action, because, in fact, there is no current between the shell or siphons and 
the wall of the tube. 
