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. Tanais 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 appearanoe. 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 

 group 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 Sphaeeomidae, 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 limbs ; 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 BOPYEIDAE are a very remarkable family of Isopods, 

 which live parasitically in the branchial chamber of prawns. 



