286 



LOWER INVERTEBRATES. 



they do not form tubes, nor do they, except Xylophaga, live in wood. In Pholas the 

 valves are large and the shells are never completely closed in front. The shell is long 

 and cylindrical, and the pallial sinus reaches to the middle of the valves. The com- 

 mon name for these 

 molluscs in England 

 is piddock, but no ap- 

 pellative has gained 

 much currency here. 

 The species bore in 

 sand, clay, limestone, 

 and even gneiss. 

 Doubtless here the 

 instrument of boring 

 is the foot with its 

 hardened dermal 

 ai-mor. When we 

 consider the hardness 

 of some of the rocks 

 perforated, we can 

 scarcely realize that 

 this organ is suffi- 

 cient to produce the 

 effects, but time is a 

 matter of small im- 

 portance to the pho- 



lads. As they increase in size they increase the size of the burrows, which are always 

 just a little larger than the shell. These burrows are always nearly vertical, and but 

 rarely encroach upon each other. 



In Europe the piddock are esteemed a delicacy, and on the coast of Normandy 

 their capture furnishes employment to a good many women and children, who pull 

 them from their burrows with an iron hook. They are usually cooked, but are said to 

 be very palatable raw. One remarkable peculiarity of the pholads is their phosphores- 

 cence or capacity of shining in the dark, which is here better developed than anywhere 

 else among the Molhisca, unless it be in Phillirhoe. The common European species is 

 Pholas dactylus, while on our coasts are found P. costata and P. truncata. A closely 

 allied genus, Zirphcea, has a species, Z. cris- 

 pata, common both to New and Old England 

 and the northern waters between. In this 

 genus the shell is short and the accessory 

 valves are absent. 



In another section of the famOy we find 

 pholads in which the anterior gape of the 

 shell is closed by a callous plate. Here be- 

 long the genera Pholadidea and Martesia. 

 Martesia smithii, on our coast, burrows in oyster shells, while other forms have differ- 

 ent habitats. While most of the Pholadidee are marine, Adams reports finding a 

 species in the fresh water of Borneo, living in dead trunks of trees. 



Fig 315 — Pholas in its burrow 



Fig. 316. — JZirphcea crispaia. 



