126 CRUSTACEA PERACARIDA CHAP. 



male are not formed inside the cases of the old styliform mandibles, 

 but are independent and possibly not homologous organs. The 

 meaning of the marked sexual dimorphism and the use of the 

 males' nippers are not in the least known, though the animals are 

 easy to keep under observation. In captivity the males never 

 take the slightest notice of either larval or adult females. 



Fam. 3. Cymothoidae. 1 This is a group of parasites more 

 completely parasitic than the foregoing, but their outer organisa- 

 tion does not differ greatly from an ordinary Isopodan form. A 

 great many very similar species are known which infest the gill- 

 chambers, mouths, and skin of various fishes. The chief interest 

 that attaches to them is found in the fact that a number of them, 

 and perhaps all, are hermaphrodite, ea*ch individual acting as a 

 male when free-swimming and young, and then subsequently 

 settling down and becoming female. This condition is exactly 

 the same as that occurring universally in the great group of 

 parasitic Isopoda, the Epicarida, to be considered later. There 

 is no evidence that the Cymothoidae are phyletically related to 

 the Epicarida, so that the similar sexual organisation appears 

 to be due to convergence resulting from similar conditions of life. 

 The general question of hermaphroditisrn in the Crustacea has 

 been shortly discussed on pp. 105-106. Cymothoa. 



Fam. 4. Cirolanidae. In this family is placed the largest 

 Isopod known the deep-sea Bathynomus giaanteus, found in 

 the Gulf of Mexico and the Indian Ocean, sometimes measuring 

 a foot long by four inches broad. A common small littoral form 

 is Cirolana. 



Fam. 5. Serolidae. 2 The genus Serolis comprises flattened 

 forms bearing a curious resemblance to Trilobites, which Milne 

 Edwards considered more than superficial. The genus is confined 

 to the littoral and deep waters of the southern hemisphere. 



Fam. 6. Sphaeromidae. 3 These are flattened, broad-bodied 

 forms, most commonly met with in the Mediterranean and warmer 

 seas. Without being actually parasitic, they are frequently 

 found as scavengers in decaying material, and they show some 

 relationship to the parasitic Cymothoidae. In some of the genera, 

 e.g. Cymodoce, the ovigerous female shows a degenerate condition 



1 Mayer, Mitth. Zool. Stat. Neapel, i., 1879, p. 165. 



- Beddard, Challenger Reports, vol. xi., 1884. 

 " Hansen, Quart. J. Micr. Sci. xlix. , 1906, p. 69. 



