ARTHROPODA 297 
The testes lie in the five posterior thoracic segments. They 
are independent of one another, and consist of three sacs, which 
open one after another into a narrow vas deferens, the latter 
ends in an elongated opening on the last thoracic segment 
(Fig. 172). 
The Isopoda are carnivorous ; many of them are parasitic, 
but seldom completely endoparasitic, living chiefly on the skin 
or in the mouth of fishes, or in the branchial chamber of 
Decapods. The group Anisopoda, which includes the genus 
Tanais, approaches most nearly the Amphipods. TZanais has 
a small carapace, which includes the first of the thoracic 
segments which bear walking legs; its body is not much 
depressed, and generally it has an Amphipod appearance. ‘The 
abdominal legs are adapted for swimming, and do not function 
as gills. The heart also lies in the thorax. 
Two different forms of male are described in Tanais dubius : 
one provided with a great developement of olfactory hairs on 
the first antennae, the other with large clasping append- 
ages. 
The other group of the Isopoda, the Euisopoda, have a 
relatively small abdomen, which carries branchial feet. The 
eroup contains several families, some of which are parasitic, 
and some of which are terrestrial in their habits. 
The CYMOTHOIDAE are partly parasitic on fish, partly free- 
living; their mouth parts are adapted either for biting or 
sucking. They are remarkable amongst Malacostraca for being 
hermaphrodite and protandrous, the young animals producing 
spermatozoa, the older animals ova. 
The SPHAEROMIDAE, which live in salt and_ brackish 
water, are interesting from their habit of rolling themselves up 
in a ball like the Oniscidae or wood-lice. The IDOTEIDAE have 
their last abdominal appendages modified to form an operculum, 
which protects the preceding branchial hmbs; the same pair 
of appendages in the ASELLIDAE are styliform, and project back- 
ward; many of the last-mentioned family are freshwater 
inhabitants ; one species bores holes in wood submerged in 
the sea. 
The BopyrIDAE are a very remarkable family of Isopods, 
which live parasitically in the branchial chamber of prawns. 
