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..—-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, each 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 [sopoda, 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 hermaphroditism in the Crustacea has 
been shortly discussed on pp. 105-106. Cymothoa. 
Fam. 4. Cirolanidae.—In this family is placed the largest 
[sopod known—the deep-sea Bathynomus giganteus, 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.’—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.’—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. 
