CHAP. VIII. RETROGRADE METAMORPHOSIS. 73 



crorum remarkably long and furnished with a strongly 

 thickened hand and a peculiarly constructed chela ; in 

 Entoniscus Porcellanae very short, imperfectly jointed, 

 and with a large ovate terminal joint (fig. 40). 



Some Isopods undergo a considerable change imme- 

 diately before the attainment of sexual maturity. This 

 is the case with the males of Tanais which have already 

 been noticed, and, according to Hesse, with the Pra- 

 nizse, in which both sexes are said to pass into the form 

 known as Anceus. But Spence Bate, a careful observer, 

 states that he has seen females of the form of Praniza 

 laden with eggs far advanced in their development. 



In this order we meet for the first time with an 

 extensive retrograde metamorphosis as a consequence 

 of a parasitic mode of life. Even in some Fish-lice 

 (Cymothoa) the young are lively swimmers, and the 

 adults stiff, stupid, heavy fellows, whose short clinging 

 feet are capable of but little movement. In the Bopy- 

 ridae (Bopyrus, Phryxus, Kepone, &c., which might 

 have been conveniently left in a single genus), which 

 are parasitic on Crabs, Lobsters, &c., taking up their 

 abode chiefly in the branchial cavity, the adult females 

 are usually quite destitute of eyes ; the antennae are 

 rudimentary ; the broad body is frequently unsymme- 

 trically developed in consequence of the confined 

 space ; its segments are more or less amalgamated with 

 each other ; the feet are stunted, and the appendages 

 of the abdomen transformed from natatory feet with 

 long setae into foliaceous or tongue-shaped and some- 

 times ramified branchiae. In the dwarfish males the 



