MOLLUSCA. 415 



well developed foot, which they possess; in other species 

 the foot is grooved; and being associated with a gland 

 which has the power of secreting a glutinous substance, 

 the latter is drawn out into slender threads, with a 

 sucker-like or flattened extremity, by which they attach 

 themselves to rocks. The grooved foot is then withdrawn, 

 and the thread hardens into an elastic sort of cord, called 

 a byssus. It is by an aggregation of these threads that 

 the common Mussel moors itself securely. The hinge of 

 the shell is formed of variously shaped teeth and depres- 

 sions; those under the beak are called cardinal teeth; 

 those on either side are lateral teeth. They are all 

 dwellers in deep water, and require to be sought for with 

 the dredge. 



The Pholas belongs to the family Pholadidce, a series of 

 animals that are remarkable for their destructive boring 

 propensities. The Teredo, shipworm, is well known for 

 the damage it does to the bottoms of ships, especially in 

 the tropical seas. Others of this family give a preference 

 to sand-stone, and even the most compact marble has been 

 found bored through by them. 



Mr. J. Robertson says : " Having, while residing here, 

 (Brighton,) opportunities of studying the Pholas dacty- 

 lus, I have endeavoured during the last six months to 

 discover how this mollusc makes its hole or crypt in the 

 chalk, by a chemical solvent ? by absorption ? by ciliary 

 currents? or by rotatory motions? My observations, 

 dissections, and experiments set at rest controversy in 

 my own mind. Between twenty and thirty of these crea- 

 tures have been at work in lumps of chalk in sea water in 

 a finger glass and a pan, at my window, for the last three 

 months. The Pholas dactylics makes its hole by grating 

 the chalk with its rasp-like valves, licking it up when pul- 

 verized with its foot, forcing it up through its principal or 

 branchial siphon, and squirting it out in oblong nodules. 

 The crypt protects the Pholas from Confervce, which, when 

 they get at it, grow not merely outside, but even within 

 the lips of the valves, preventing the action of the siphons. 

 In the foot there is a gelatinous spring, or style, which 

 even when taken out has great elasticity, and which seems 

 the mainspring of the motion of the Pholas dactylus." 1 



(1) See Addenda. 



