1909.] FKOM THE BAY OF BISCAY. 849 



which appear to prove conclusively that not only do these animals 

 become sexually mature before attaining their full growth, 

 but that in both sexes the secondary sexual characters undergo 

 considerable modification aftet' sexual maturity is reached, the 

 character most affected being the second gnathopod. Hitherto it 

 has been accepted as a general rule amongst Amphipods that the 

 male only has been modified, and that it is subject to variation in 

 a far greater degree than the female ; but an examination of this 

 material shows that the female undergoes quite as much modi- 

 fication as the male, though owing to the usually smaller size of 

 its gnathopods, the changes are less noticeable. (For an examj)le 

 of diversity in the shape of the gnathopod hand of an ovigerous 

 female see Symjyleustes grandimanus, PL LXXX, figs. 11 & 12.) 

 In some species the degree of difterence between the stages of 

 development is negligible ; in others it is of remarkable extent, 

 so much so that the two extremes of a series of stages might be 

 taken for absolutely distinct species. Further knowledge on this 

 point will lead to a considerable revision of specific nomenclature. 



The variation in the gnathopod of the male has been noted 

 by many writers, the first to discuss it being Fritz Miiller (30) 

 pp. 16-18. Miiller figured two forms of the hand of the male of 

 Orchestia darivinii, Siud stated that there were "nur zwei durch 

 keinerlei Zwischenglieder verbundene Formen." He considered it 

 a case of true dimorphism. Darwin quoted Midler's observations 

 in support of his argument for Sexual Selection, (12) vol. i. p. 332, 

 vol. ii. p. 215. Faxon also refers to them, (14) p. 43, (15) 

 pp. 12 and 111, and suggests as explanation, that the two forms 

 may be, as in Camharas, alternating periods in the life of the 

 individual, one form being assumed during the i3airing season, 

 and the second form during the intervals between the pairing- 

 seasons. Delia Valle (13) p. 508, attributes the differences in 

 the two forms of the male of Orchestia darwinii to development ; 

 so also Stebbing (40) p. 545, where the animal is described from 

 Midler's drawings. Geoftrey Smith (33) in a series of obser- 

 vations on Orchestia deshayesii and 0. gammarellus has made 

 the important discovery " that the males of these species, when 

 breeding is not going on, assume a semi-hermaphrodite condition 

 of a quite indubitable kind " ; and that it is during this period 

 of sexual suppression that active growth takes place. He states 

 (p. 91) that "the antagonism that exists between the functional 

 activity of the sexual organs and growth of the body . . . appeai-s 

 to result in the phenomena of high and low dimorphism only in 

 the male sex.'' It should be noted, however, that Boulenger (5) 

 has recently questioned these conclusions. 



The development of the female would appear to proceed on 

 totally different lines, and to follow a steady course from im- 

 maturity to full growth. In one species, Rhachotropis helleri 

 Boeck (infra, p. 869), I was able to examine a large number of speci- 

 mens, about 250, and to trace a continuous series of stages of 

 development. The stages are easily observed in this species because 



