ARTHROPODA 293 



from the system, and although they open close to its end, they 

 belong to the mid gut and not to the proetodaeum. 



The abdomen is reduced to a limbless process in the ' 

 Laemodipoda, a group which includes Cyamus ceti parasitic on 



Pig. 169. — Gammarus neglectus. Section through the 

 third thoracic segment. From Leuckart and Nitsche, 

 after G. 0. Sars. 



1. The heart. 



2. The alimentary canal. 



3. The ovary. 



4. Tlie hepatic diverticula. 



5. The nervous system. 



6. The epimeron. 



7. The thoracic appendage bearing — 



8. The lamina forming the hrood-pouch (postegite), and 



9. The branchiae. 



the skin of whales, and in the family Capeellidae which 

 comprises curious elongated thin animals of a very grotesque 

 appearance, mostly found living amongst Polyzoa and Hydroids. 

 The family Pheonimidae includes some forms which have an 

 enlarged head ; the female Phronima is a good deal larger than 

 the male, and is usually found swimming about in a barrel- 

 shaped investment formed by the transparent colonies of the 

 compound Ascidian Pyrosoma ; other allied forms live in 

 jelly-fish. 



Sub-order 2. ISOPODA. 



Chaeacteeistics. — Arthrostraca with usually a dorso-ventrally 

 depressed body ; thorax of six or seven free segments ; ahdomen 

 often reduced, it hears lamelliform, appendages, the endo- 

 podites of some of which function as gills. 

 A very typical example of an Isopod is presented by the 

 little brownish-gray animal Asellus aquaticus, which inhabits 

 lakes, large ponds, and marshes. Its body is flattened, and 

 composed of three regions — the head, the thorax, and the abdo- 

 men; of these the thorax is far the largest, and consists of 

 seven free segments and one fused with the head. The abdom- 

 inal segments have all fused together with the exception of 

 the first, which is very small, and bears rudimentary appendages. 



