1060 MR. CYRIL CROSSLAND O^ 



outer shell-layers only, and as both of thase are weak and horny, 

 so are the processes. These consequently get worn oft' the older 

 parts of the shell, and as increase in diameter becomes slow so 

 does the development of these processes * and in full-grown 

 shells practically ceases altogether. 



It is obviously an advantage to the " oyster " to grow up 

 rapidly, and form a shell large enough to resist teeth and pincers 

 as quickly as possible. Margaritifera vulgaris succeeds excellently 

 in this, attaining nearly its full size in a year ; but it does this at 

 the expense of the sooner losing its shell-processes and so being 

 open to attack from Murex, and doubtless other predatory 

 prosobranchs. 



In M. ')nargaritifera also growth is extremely rapid during 

 the first year, and at the end of it the shell is strong enough 

 to resist such ci-abs and ordinary fishes as infest this bay ; but 

 after the second year it is slow, a shell six years old being far 

 from full-sized. Consequently the formation of marginal pro- 

 cesses continues later, and with it partial immunity from attack 

 by Mibrex. Sexual maturity is reached by all these species alike 

 in the second year. 



From the I'esult of my experiments recorded above, one wonders 

 how the first three species can possibly survive in Natvire. The 

 explanation is very simple : they choose a habitat out of rea,ch 

 of their foe. Ostrea sp. ? and Margaritifera mauritiif inhabit 

 crannies' among other shells + or in stones, too narrow for Murex 

 to enter ; while Avicula zebra, like other species of the same 

 genus and a few other genera of the same family, possesses a 

 foot that fears not the stings of corals, nor even those of Mille- 

 pora, and so attains a habitat inaccessible to all others (text- 

 fig. 230, p. 1058). 



Avicula zebra is very common on Millepora alcicornis in the 

 Red Sea ; another advantage of this habitat is support above the 

 sea-bottom, clear of all obstructions to its respiratory circulation, 

 which latter is of greater value than would be supposed. 



Given protection fi'om enemies, and from being smothered in 

 mud, all these species will flourish wherever the experimenter 

 likes to place them, these two considerations being all that 

 causes the restriction of their habitats in nature. 



* Only in a general sense is tliis true. In two specimens of M. margaritifera 

 of about the same age, viz. two years, one has added to its diameter much less 

 than has the other, owing to difference of conditions in which they grew ; but this 

 has not hindered development of processes, which are merely nearer. together in 

 the slower-growing specimens. It is when slow growth is due to age that the 

 formation of processes becomes imperfect. 



t This species, when over a j'ear old, forms a postero- ventral non-nacreous addition 

 to the shell, similar to, but smaller than, tliat found in many other Aviculidaj, 

 and which reaches extreme development in Slalleus. In all, the addition is an 

 adaptation to enable the animal to inhabit narrow crevices, and its function is 

 similar to that of the siphons of those lamellibranchs which live bui'ied in sand, or 

 in burrows. 



X 1^-ff; on the under sides of large specimens of mother-of-pearl shell brought in 

 bv the divers. 



