ISOPOPODA CHEL1FERA SEXUAL DIMORPHISM 123 



Heterotanais, etc.) live in the algal growths of the littoral zone, 



and being highly heliotropic they are 



easy to collect if a hasinful of algae is 



placed in a strong light. The females 



carry the eggs about with them in a 



brood-pouch formed, as is usual in the 



Peracarida, by lamellae produced from 



the bases of the thoracic limbs. The 



males on coming to maturity do not 



appear to grow any more, or to take 



food, their mouth -parts frequently 



degenerating and the alimentary 



canal being devoid of food. They 



are thus in the position of insects 



which do not moult after coming to 



maturity ; and, as in Insects, the 



males are apt to show a kind of 



high and low dimorphism certain of 



the males being small with secondary 



sexual characters little different from 



those of the females, while others are 



large with these characters highly 



developed. Fritz Miiller, in his 



Facts for Darwin, observes that in a 



Brazilian species of Leptoclielia, ap- 



parently identical with the European 



L. duliia, the males occur under two 



totally distinct forms one in which 



the chelae are greatly developed, and 



another in which the chelae resemble 



those of the female, but the antennae 



in this form are provided with far 



longer and more numerous sensory 



hairs than in the iirst form. Miiller FIG. 81. Apseuttf* *t>iiiosus, 6. 



suggested that these two varieties 



were produced by natural selection, 



the characters of the one form 



Ab 



x 15. 



tith al 



S;ir>. 



J , 1 "! .UltrlllNi : .1'', 



a]i]irliil;i_'r : '/'. 

 A I'ti'l 



pensating for the absence of the characters of the other. A 

 general consideration of the sexual dimorphism in the Tanaidac 



1 Smith. Millli. '/<,!. S/,i>. Neapcl, xvii., 190;'.. j,. yr_'. 



