16 



C, M. YONGE 



vi. SuPERFAMiLY Cardiacea. Family Tridacnidae. THdacna cro- 

 cea. This species, confined to the mid-tropics in the Indo-Pacific, 

 where it is often extremely abundant, boring into coral boulders, is 

 perhaps the most unexpected of bivalve borers. It belongs to a 

 family which includes the largest bivalve ever evolved, THdacna 

 deresa, the giant clam. All species in the family begin life attached 

 by byssus threads. In the larger ones the bvssus and the shell gape 

 through which it projects disappear and the animals finally main- 

 tain themselves by their unaided weight. But in T. crocea the 

 byssus, and the gape through which it emerges, become larger with 

 age, and at the same time the animal proceeds to bore doivnward 

 into the calcareous rock (Fig. 7). This is a consequence of the 



Fig. 7. Tridacna crocea, viewed, from left side, in situ within boring in 

 coral rock, shown in longitudinal section. Animal lies with hinge (H) under- 

 most, massive byssus (obscured by shell) attached to pillar (R) anterior to 

 this. A, P, anterior, posterior. (After Yonge, 1936.) 



unique form of these animals. The shell valves have changed their 

 position in relation to the enclosed body so that the hinge comes 

 to lie beside the byssal gape on the underside of the body. The 

 result is that the siphonal tissues (normally at the posterior end) ex- 

 tend along the upper, i.e. dorsal, surface. They also extend laterally, 

 forming a broad band of brilliantly pigmented tissues facing up- 

 ward to the light. Within these tissues are contained countless mil- 



