ISOPOPODA CHELIFKRA SEXUAL DIMORPHISM I 23 



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

 and being highly heliotropic they are 

 easy to collect if a basinful 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 

 decfeneratinw 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 Tteing 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 Leptochelia, ap- 

 parently identical with the European 

 L. duhia, 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 tlie female, but the antennae 

 in this form are provided with far 

 longer and more numerous sensory 

 hairs than in the first form. Miiller Fig 



suggested that these two' varieties 



81. — Apseudes spinosus, 6 . 

 X 15. .1, Ist antenna; .1''. 

 tjtli abdominal appendage : T. 

 '2nd thoracic appendage. 1 After 

 Sar.s.) 



were produced by natural selection, 

 the characters of the one form com- 

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

 general consideration of the sexual dimorphism in the Tanaidae ^ 



1 Sniitli. Milth. ZooJ. Stat. Xcapc/, xvii., 190.'. p. 312. 



