MOLLLSCA. 531 



the tentacles on the borders of the mantle. In other 

 genera, the eyes are differently placed, in Pinna on the 

 fore part of the mantle, and around the siphon-orifices in 

 Pholas and Solen. In the Cockle (Cardium) the short 

 siphons are surrounded with an extraordinary number of 

 tentacles, capable of protrusion, each of which bears a 

 pretty little eye ; these are beautiful objects under the 

 microscope. Cockles are able to perform vigorous leaps by 

 means of a 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 gluti- 

 nous 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 is then withdrawn, and the thread hardens into an elastic 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 muors itself 

 securely. The hinge of the shell is formed of variously 

 shaped dentations ; those under the beak are called car- 

 dinal teeth ; those on either side are lateral teeth. 



The Pholadidce are a series of animals remarkable for 

 their destructive boring propensities. The Teredo, ship- 

 worm, 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 sandstone, and even the 

 most compact marble has been found bored through by 

 them. 



Mr. J. Eobertson says : — " Having, while residing here 

 (Brighton), opportunities of studying the Pholas dactylus, 

 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 1 by absorption 1 by ciliary currents 1 

 or by rotatory motions % My observations, dissections, and 

 experiments set at rest controversy in my mind. Between 

 twenty and thirty of these creatures 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 dac- 

 tylus makes its hole by grating the chalk with its rasp-like 

 valves, licking it up when pulverized with its foot, forcing 

 it up through its principal or branchial siphon, and 

 squirting it out in oblong nodules. The crypt protects the 



m m 2 



