THE EDRIOPHTHALMIA. 313 



The seven free somites of the thorax each give attachment 

 to a pair of limbs. It is characteristic of Amphitho'e, as of the 

 Amphipoda in general, to have the five anterior pairs of tho- 

 racic members directed forward. Each limb consists of an 

 expanded coxopodite, succeeded by the other six joints of the 

 typical crustacean limb. 



In the male, a single vesicular lamella, the branchia, is 

 attached to the inner side of the coxopodite of the append- 

 ages of the ninth to the fourteenth somites inclusively ; but, 

 in the female, an additional plate, convex externally and con- 

 cave internally, is attached above, and internal to, the branchia 

 of the 9th to the 12th somite. These oostegites, as they may 

 be called, inclose a cavity in which the incubation of the eggs 

 takes place. 



The abdomen consists of six somites and a very small ter- 

 minal telson. The appendages of the three anterior somites 

 are terminated by two multiarticulate setose filaments (Fig. 

 81, XV), while in the three posterior the corresponding 

 parts are styliform, and serve as a fulcrum for the abdomen 

 when the animal leaps, by the sudden extension of that region 

 of the body. 



The Edriophthalmia are ordinarily divided into three 

 groups. The Amphipoda, which resemble Amphitho'e, are 

 characterized by their compressed form and their ordinarily 

 saltatory habits ; by having thoracic branchiae ; by the for- 

 ward direction of their four anterior locomotive limbs (2d 

 to 5th pairs of thoracic appendages), and by the contrast 

 between the three anterior and the three posterior pairs of 

 abdominal appendages. The common Sand-hopper is the most 

 familiar example of this division. The second group is that 

 of the Lwmodipoda, distinguished by the rudimentary state 

 of the abdomen, which is reduced to a mere papilla, and by 

 the coalescence of the second, as well as the first, thoracic 

 somite with the head, so that the anterior limbs appear to be, 

 as it were, suspended under the neck. The strangely-formed 

 genera Cyamus, the parasite of whales, and Caprella, which 

 is very common upon our own coast, adhering to corallines, 

 sea-weeds, and starfish, belong to this group. 



The Isopoda, which constitute the third group of the 

 Edriophthalmia, are usually depressed instead of compressed, 

 and run or crawl instead of leaping. Many, like the common 

 Woodlouse (Oniscus), possess the power of rolling them- 

 selves into a ball when alarmed ; some, like the last-named 

 genus, are terrestrial ; others, like the Asellus, inhabit fresh 

 U 



