312 TRANSACTIONS OF THE [aPR. 12, 



Very much has been written about the method "of boring. In 

 1888* it was maintained that in Pholas and Lithodomus the foot 

 was reinforced by siliceous particles. Many have looked to the 

 action of chemical agents secreted by the animal, but these have 

 failed of detection. It is now very generally thought, and it 

 may be maintained with surety, that the boring is purely a me- 

 chanical process, at least in the Teredo, Pholadidse, Pai^apholas 

 and their allies, and that siliceous particles play no part. I can- 

 not speak of the habits of LitJwdovius from personal observation, 

 but the facts in the case of Adida falcata would seem to throw 

 doubt on the ability of Lithodomus to bore. It is commonly 

 supposed that Adula is a borer, and the circumstantial evidence 

 partly supports the view. However, if the animal is examined 

 in its home, it will be found to occupy the burrows of Pholadidse 

 or other boring clams and to by no means fill the burrow. It is 

 moreover always attached by a byssus to the wall of the burrow 

 near its open end, out of which the posterior end of the valves 

 protrude. Furtheimore, Adula has a musculature and siphon in 

 every way comparable to that of the mussels, and it, like the 

 latter, has a reduced foot which is in no way comparable to that 

 of the piddock. Nor does the shell present any such characters 

 as those which are coordinate with the musculature of the 

 Pholadidse, but is covered with a thick and unbroken' perios- 

 tracum. That it does, however, make use of the excavated 

 homes of other forms is undoubted, and it is not imi)robable that 

 it sometimes usurps the right of the owner and finally causes its 

 death by starvation. So very similar to the structure of Adula 

 is that of Lithodomus that the suspicion is raised that the latter 

 ma}' not burrow at all. At any rate, the matter should be further 

 studied. 



The tools for boring in the Pholads consist of the foot and 

 four adductor muscles as the seats of motor power ; and the an- 

 terior part of the shell, as cutting instrument. The successive 

 layers of the fore part of the shell are plaited into tooth-like pro- 

 jections which in the younger specimens of Zirphsea are fine but 

 strong. 



In the work of boring the first movement is the extension of 

 the foot so as to apply it to a part of the base of the burrow. 

 Clinging as a sucking disc, the contraction of the foot muscles 

 now causes the anterior region of the shell to be closely applied 

 to the wall of the burrow. This movement is anticipated by the 

 contraction of the anterior part of the anterior adductor and of 

 the ventral adductor, the result of which is to approximate the 

 front margins of the shell. The teeth are now in a position 



*Vogt & Young, " Traite d'Anatomie Comparee Pratique." Paris, 1888. 



