Vv ISOPOPODA—_-CHELIFERA 
SEXUAL DIMORPHISM 123) 
Heterotanais, ete.) 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 hmbs. 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 Miller, in his 
Facts for Darwin, observes that in a 
Brazilian species of Leptochelia, ap- 
parently identical with the European 
L. dubia, the males oceur 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 first form. Miiller 
suggested that these two varieties 
were produced by natural selection, 
the characters of the one form com- 
Fic. 81.—Apseudes spinosus, 6, 
x 15. A, Ist antenna; Ad, 
6th abdominal appendage; 7’, 
2nd thoracic appendage. (After 
Sars. ) 
pensating for the absence of the characters of the other. <A 
general consideration of the sexual dimorphism in the Tanaidae ' 
1 Smith, Mitth. Zool. Stat. Neapel, xvii., 1905, p. 312. 
