198 



HIGH SCHOOL ZOOLOGY. 



that the four hindmost abdominal segments 

 are coalesced above into a shield, from beneath 

 which the last pair of legs project. The three 

 pairs of legs in front of these serve for respira- 

 tion, and the eggs are carried in the female on the 

 under surface of the thoracic segments. Terrestrial 

 Isopods, like the common Wood-louse (Oniscus) 

 and its allies, exhibit an interesting adaptation for 

 breathing air ; one of the pairs of abdominal legs 

 being traversed by tubes which have the same 

 function as the tracheae of insects. 



Among the marine Isopods several are temporary par- 

 cummunitxz.'^ asites adhering to the surface of fish ; others are perman- 

 ent parasites, which live in the gill- or body-cavity of 

 other Crustacea, and which consequently loose much of their resem- 

 blance to the free Isopods. 



Of the fresh-water Amphipods species of a genus Gammarus 



(Fig. 124), are everywhere to be 

 50 //^v^Xj -('.>^ /\ meb with. The gills are on the 



thoracic legs, the abdominal legs 

 being partly for swimming, and 

 partly for leaping. Species of an 

 allied genus, Pontiporeia, occur in 

 Flgr. 124.-Ga7«»i«r«8 8p. x 3. *^«^ Great Lakes; they are inter- 

 esting, like Mysis, because the other species ai'e chiefly marine. 

 14. The lower orders (separated in a sub-class Entomostraca, 

 from the foregoing Malacostraca,) exhibit by no means the same 

 constancy in the number of segments which we meet with in 

 the higher, nearly allied forms often presenting considerable 

 differences in this respect. All of the orders except one — the 

 Cirripedia — have fresh- water representatives, which are for the 

 most pari; inconspicuous, often microscopic, creatures. 



The most primitive forms, as well as the largest we Lave to 

 mention, belong to the Phyllopoda, a group in which all the 

 segments behind the head bear flat leaf-like swimming-leas. 



