CRUSTACEA 273 



to the other sex also ; the fact that they are not is only 

 intelligible on the view that they are produced by the 

 cramping influence of the tubular habitation, and are there- 

 fore correlated with the sex which alone is subjected to that 

 influence. 



A great difference between the sexes occurs also in another 

 Isopod, Gnathia, Leach, the males having for a long time been 

 distinguished as Anceus, and the females as Praniza. 



There are several species of Gnathia, but it is not necessary 

 here to consider the differences between them. It will be 

 sufficient to take one of the commonest, namely Gnathia, 

 maxillaris, as an example. Sars 1 states that this species 

 occurs abundantly off the coasts of Norway on muddy ground 

 at depths from 20 to 100 fms. The males are more commonly 

 taken than the females, and both are found slowly creeping 

 on the sea-bottom. They are also to be found between tide- 

 marks on the British coasts. 



The body of an Isopod is usually described as consisting 

 of three regions the cephalon, composed of the fused segments 

 which bear the feeding appendages, the pereion, including 

 the segments which bear the crawling legs, and the pleon, 

 composed of the segments which bear the swimming appendages. 

 The differences between the adult male and female Gnathia 

 occur only in the cephalon and pereion. In the male (Fig. 

 30, A, B) the cephalon is broad, flattened, and truncated in 

 front, and from this front edge project a pair of relatively very 

 large strong appendages, usually regarded as the mandibles. 

 Besides these there are visible from the dorsal side two short 

 pairs of antennae The oral appendages of a typical Isopod 

 include, besides the mandibles, two pairs of maxillae, and 

 one pair of maxillipeds or gnathopoda. In the adult male 

 Gnathia maxillae are wanting, but there is a pair of large 

 triangular appendages covering the oral region, and beneath 



1 Crustacea of Norway, Bergen, 1897, vol. i. "Isopoda," p. 51. 

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