480 Mr. J. H. Orton. Occurrence of Protandric [June 8, 



Chain-formation, along with protandric hermaphroditism, effects a strict 

 economy of the sperm of the males, since the sperm is probably all transferred 

 to the females ; moreover, this arrangement probably ensures cross- 

 fertilisation of all the females. In this respect chain-formation with 

 protandric hermaphroditism is an advantage over permanent hermaphroditism 

 with self-fertilisation, and no doubt leads to as great productiveness as would 

 obtain in a motile unisexual condition. Crepidula fornicata, therefore, has 

 become adapted to a sedentary life without losing any of the procreative 

 advantages of a free -living habit. 



The individuals forming side-chains are almost always on the left side 

 of the primary chain, and therefore cut off from all sexual relations with 

 any of the individuals of the latter. Here would appear at first sight to be 

 a mal-adaptation, but individuals settling on a primary chain are quite 

 comparable with those settling down on foreign objects ; if the phenomenon 

 is not due merely to chance, however, the former would appear to be the more 

 gregarious. 



Occurrence, of Dwarf Females. 

 The variation in size in this species is indeed remarkable ; it has been well 

 described by Prof. Conklin (5, pp. 438 to 440). Size, besides being dependent 

 on the usual conditions, is also determined by the extent of the surface 

 of attachment. If individuals settle down on a small pebble or other surface 

 where expansion is impossible, they remain permanently dwarfed. Mature 

 dwarfed females have been found in such situations as small as 1*8 cm. long by 

 1 cm. wide, their shells being generally somewhat thickened, especially 

 around the edge. On a flat surface of unlimited extent individuals may 

 grow to a size of 5'5 cm. long by 3'5 cm. wide. Crepidula fornicata is thus 

 able to regulate its shell-forming metabolic processes to the individual 

 requirements. A similar readiness to adapt itself to its situation is exhibited 

 by C. plana; but an even greater difference in the size of the extremes has 

 been observed in this species by Prof. Conklin (2, p. 12), a race of dwarfs 

 having been described by him as being one-thirteenth the size of the larger 

 forms ! 



The smallest females found in Crepidula fornicata are the dwarfs 

 mentioned above: they often occur in the middle of an oyster-shell surrounded 

 by chains, the posterior ends of the shells of which converge on the middle of 

 the oyster -shell, thus preventing expansion of the enclosed individual. 

 Dwarf females, however, often have § forms or ? p.r.'s fixed upon them, and 

 since single settled males are found quite as small, there is no doubt that 

 these small females have once been males and are really dwarfed. These 

 dwarfs, as in the case of those of C. plana, appear to be dwarfs merely by 



