MOLLUSC A. 531 



the tentacles on the borders of the mantle. In other 

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

 lore part of the mantle, and around the siphon-orilices 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 povvor 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 is then withdrawn, and the thread hardens into an elastic 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 bi/ssus. 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 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 ? by absorption ] by ciliary currents ? 

 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 wmdow, for the last three months. The PJwlas dac- 

 tylus makes its hole by grating the chalk Avith 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 



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