C, M. YONGE 



groups, both intake and extrusion of the essential water current 

 take place at the posterior end. Hence when these animals have 

 succeeded in penetrating into rock they have achieved maximum 

 protection while continuing to feed by ciliary currents in the normal 

 manner. There is no question of any of them obtaining anything of 

 food value from the rock. The shell valves are always the prime 

 agents of boring, although the manner in which they operate to 

 this end differs greatly. Only in certain members of one superfamily 

 (vii below) is the mechanical action of the shell valves undoubtedly 

 assisted by chemical means. 



Origin and Nature of Boring in the Bivalvia 



So well are these animals basically fitted for boring that members 

 of no less than seven superfamilies of the Bivalvia have, independ- 

 ently of one another, taken to this mode of life. It is interesting also 

 that they have arrived at this final habit by one of two routes. 



Primitively the Bivalvia are members of the infaima, i.e. they 

 live within soft substrates through or down into which they burrow 

 by means of the initially hatchet-shaped foot terminally dilated 

 by blood pressure. By further development of the habit of deep 

 burrowing they have come to penetrate stiffer and stiffer substrates, 

 finally becoming true borers. This is true of some or all of the mem- 

 bers of the first two superfamilies ( i and ii ) described below. 



Certain Bivalvia, e.g. marine mussels, have secondarily become 

 permanently attached in adult life to a hard substrate, i.e. they have 

 become members of the epifauna. For reasons given elsewhere 

 (Yonge, 1962), this is probably due to retention into adult life of 

 the mechanism of attachment by byssal threads. This probably was 

 originally a larval structure concerned solely with temporary at- 

 tachment of the newly settled larva, and this certainly constitutes 

 its sole function in the more primitive infaunal bivalves. The epi- 

 faunal habit brings the bivalves into intimate association with 

 rock, and this is the certain, or very probable, route whereby the 

 boring habit has been reached in superfamilies iii to vii. 



The degree of speciafization exhibited within the different groups 



