440 



ARTHROPODA. 



from some of the thoracic segments and the abdomen is reduced. Species 

 of Caprella* are common on hydroids. Cyamus ceti is parasitic on whales. 



Order II. Isopoda. 



The Isopoda are readily distinguished from the Amphipoda by 

 their depressed (i.e. horizontally flattened) bodies. The feet are 

 adapted for creeping, and a brood pouch is formed as in the Am- 

 phipoda, but gills are lacking here since some of the abdominal 

 feet are modified for respiration (fig. 451, k). In the abdomen, 

 the somites of which exhibit a great tendency to fusion, the telson, 

 as in all Malacostraca, is without appendages; the sixth somite 



FIG. 451. 



FIG. 452. 



FIG. 451. Asellus aquaticus. (From Ludwig-Leunis.) a 1 , a 2 , antennae; />r, brood 

 pouch; fc, pleopoda modified to gills; md, mandibles- p 1 -/) 7 , thoracic feet; 

 paHxt'i abdominal feet (pleopoda); I-V1, head; VII-XIII, thoracic segments; 

 XIV- XX, abdominal segments, partly fused. 



FIG. 152,Cymothoa emarginata. (After Gerstacker.) p 8 , sixth pleopod. 



bears, in the walking forms, long forked appendages (fig. 451); in 

 the swimming species (fig. 452) they are flattened and, with the 

 telson, make a swimming organ. The five anterior pairs of pleo- 

 poda are modified for respiration, by the expansion of the endop- 

 odites into thin-walled plates, while the exopodites and the whole 

 first pair serve as opercula or gill covers. As a result of this posi- 

 tion of the gills the heart (usually with two pairs of ostia) is ab- 

 dominal in position. 



In the terrestrial species the gills are adapted for breathing damp air. 

 In Porcellio and Armadillidum the first or first and second opercula are 

 permeated with a system of air tubes, which physiologically, though not 

 morphologically, are comparable to the tracheae of insects. 



In the Isopoda the tendency to parasitism is greater than in the 

 Amphipoda. Many swimming forms attach themselves to fishes and 

 feed by boring with their mouth parts, which are modified for the purpose, 



