442 ARTHROPODA. 



usually hermaphroditic, but have besides complemental dwarf males (fig. 

 453, A). 



Sub Order I. ANISOPODA. Six free thoracic segments; heart tho- 

 racic; first thoracic foot (on head) chelate; abdomen with swimming feet. 

 A group intermediate between Amphipoda and other Isopoda. Tanais,* 

 Leptochela * (fig. 454). 



Sub Order II. EUISOPODA. Seven free thoracic segments. ONISCID.E; 

 terrestrial, familiarly known as sow bugs; Ligia, on seashore; Porcellio* 

 Oniscus* ArmadilUdum* 'pill bug.' ASELLID.E (fig. 451), fresh water. 

 SPILEROMID.E, head broad, body rounded and convex; Sphwroma* Lim- 

 noria lignorum * (fig. 454), the gribble, attacks submerged wood and is 

 nearly as destructive as Teredo. IDOTEIDJE, free-living, marine, with usually 

 elongate bodies; Idotea,* Ccecidotea* BoPYRnxE, parasitic on Caridea; 

 body of female disc-like, asymmetrical, without eyes; Bopyrus* CYMO- 

 THOID^E, parasitic on fishes or in their mouths. Cymothoa* Mga* 

 Cirolana* 



Sub Order III, ENTONISCIDA, parasites whose general features are 

 described above. Entoniscus, Cryptoniscus. 



Class II. Acerata. 



The animals comprising this group were formerly divided 

 among the tracheates (p. 408) and the Crustacea, but more recent 

 studies show that, although differing widely in respiration, the 

 forms included are closely allied in structure and development and 

 present many differences from both Crustacea and from other 

 tracheates (Insecta). The former views were based upon a con- 

 fusion between analogy and homology, it being thought that 

 tracheae wherever found were homologous structures. 



In the Acerata the body is usually divided into two regions, 

 cephalothorax and abdomen, though in some cases (mites) the 

 two regions become fused. The cephalothorax consists of six 

 somites which always bear appendages, and these appendages are 

 arranged in a circle around the mouth, the basal joints of one or 

 more pairs frequently serving as jaws. None of these appendages 

 are like antennae (whence the name of the group). The abdomen 

 consists of a varying number of somites, all of which may be free, 

 or, again, may be fused into a common mass. These abdominal 

 somites bear appendages in the embryo, but in the adults (except 

 the Xiphosura) these are usually lost or so modified that their 

 existence is only recognized by a study of development. 



The alimentary canal is straight, without marked enlargements, 

 and lacks a chewing stomach. The liver is large and opens into 

 the intestine by two or more pairs of ducts. The nervous system 

 has some or all of its ventral ganglia arranged in a ring around the 



