PARASITES AND MESSMATES 235 



been most fully studied in the case of a kind of 

 Spider Crab common at Naples Inachus mauritanicus 

 In this species it is found that females infected with 

 Sacculina show no conspicuous external modification, 

 except that the abdominal appendages, which in the 

 normal females serve for the attachment of the eggs, 

 are greatly reduced in size. Infected males, how- 

 ever, may assume to a greater or less degree the 

 characters proper to the female sex. Some males 

 show little change, except that the chelipeds remain 

 small and flattened, as in the females and non- 

 breeding males. Other specimens have, in addition, 

 the abdomen much broader than in normal males, 

 and sometimes as broad as in the females. Finally, 

 some males develop on the abdomen, in addition to 

 the rod-like appendages on the first and second 

 somites, characteristic of the male sex, two-branched 

 appendages on the next three somites, as in the 

 females ; these individuals are, in fact, so completely 

 intermediate in character between the two sexes that 

 it is only by dissection that it is possible to recognize 

 them as modified males. 



An indication of the way in which the degenerate 

 Rhizocephala have been derived from normal Cir- 

 ripedes is given by a peculiar species of pedunculate 

 Barnacle, Anelasma squalicola, which lives attached to 

 Sharks and Dogfish in the North Sea. In Anelasma 

 the peduncle becomes deeply buried in the flesh 

 of the Shark, and its surface is covered with short 



