298 ZOOLOGY 



They are dioecious and markedly dimorphic, the dwarf male 

 living attached to the body of the female. The female is 

 somewhat disk -shaped, asymmetrical, and with but slight 

 traces of segmentation, and no eyes. The male is elongated, 

 segmented, and provided with eyes. The mouth parts are 

 rudimentary ; in the female the seven thoracic legs bear 

 oostegites, which form a brood-pouch. In those species with 

 a piercing mouth the upper and lower lips form a suctorial 

 tube, within which the mouth parts are enclosed in the form 

 of piercing stylets. A very remarkable fact connected with 

 the parasitism of the Bopyridae is that the presence of certain 

 species of this group has the effect of materially altering the 

 reproductive organs of its male host. For instance, a male 

 Pagurus infested by Phryxus paguri has its abdominal append- 

 ages of the female type, and its testis is degenerate and the 

 spermatozoa imperfect. The same is true for many other 

 species. 



In the family Entoniscidae the more complete parasitism 

 has involved a greater departure from the ordinary Isopod 

 type. Their body is a limbless sack, which lives in an in- 

 vagination pushed into the bodies of Cirrhipedes, Paguridae, and 

 Crabs. The invagination retains its opening to the exterior. 

 The sexual forms of Portunion are very remarkable ; the 

 large deformed parasite is a protandrous hermaphrodite, 

 functioning in its adult stage as a female; besides this form 

 there are small males which remain, in a larval stage, and also 

 degraded complemental males. It seems that out of the 

 numerous larvae, those which obtain the most advantageous 

 position in the host will become the hermaphrodites and will 

 ultimately produce eggs ; those which occupy the next best 

 position become the males, which retain their larval features, 

 whilst the most disadvantageously situated larvae degenerate 

 and form the complemental males. The genus Cryptoniscus is 

 commonly parasitic on the Ehizocephala, which are themselves 

 parasitic on other Crustacea; it is stated that if the Ehizo- 

 cephala die the Isopods still continue to receive nutriment 

 through the root-like processes of their dead hosts, as if they 

 were parts of its own body. 



The family Oniscidae, the wood-lice, have become terres- 



