l62 



SYSTEMATIC ZOOLOGY 



readily distinguished from the second 

 order of Arthrostraca by the general 

 shape of the body, which is decidedly 

 flattened dorsoventrally. The seven 

 pairs of walking appendages on the tho- 

 rax are all very much alike, and in the 

 female some of them have special plates 

 on the inner side, which serve to hold 

 masses of eggs in place against the 

 ventral wall of the body, where they 

 remain during a considerable period of 

 their development (Fig. 153). The 

 abdominal appendages are more or less 

 platelike and most of them function as 

 gills. Most of the Isopoda are free-living, but some are more or 

 less permanently parasitic on the skin and in the mouth of fishes; 



FlG. 154. Porcellio scaler, a 

 wood louse, enlarged about 

 two diameters. (From Ship- 

 ley and MacBride's Zoology.) 



23 



,-XJVJfllf 



FlG. 155. Gammarus neglectus. Female with ged. I-VI, cephalothorax VII— 



XII I, free thoracic segments; XIV-XIX, the six abdominal segments. I, anterior antenna; 

 2, posterior antenna; 3, mandible; 4, firsl maxilla; 5, second maxilla; 6, maxilliped ; 7- 

 13, thoracic legs; 14-16, three anterior abdominal legs for swimming; 17-19, three posterior 

 abdominal legs for jumping; 20, heart with six pairs of ostia; 21, ovary ; 22, hepatic diver- 

 ticula; 23, posterior diverticulum of the alimentary canal; 24, median dorsal diverticulum; 

 25, alimentary canal; 26, nervous system; 27, ova in brood pouch, formed from lati 

 on the ond, third, and fourth thoracic legs (After Sars, from Shiple> and 



MacBride's Zoolog 



