222 INVERTEBRATE ZOOLOGY 



springing organs. Gammarus and Hyallela are common fresh- 

 water genera, while Orchestia (Fig. 104) and Caprella are marine. 



VI. Order Isopoda 



A dorsoventral flattening of the body is characteristic of 

 most isopods. The first thoracic segment is coalesced with the 

 head. Though the body superficially shows sharp marks of 



segmentation, there is a tendency for 

 the abdominal somites to fuse. All of 

 the abdominal appendages are similar 

 and usually biramous. Gills are borne 

 on the ventral side of the abdomen and 

 in the terrestrial species are capable of 

 utilizing moist air for respiration. A 

 brood sac is borne ventral to the thorax 

 as in the amphipods. 



Isopods, or sow bugs or pill bugs as 

 they are called popularly, are found 

 Fig. 105.— a fresh-water in both fresh-water (Asellus, Fig. 105) 

 \7fSr°Kunket^''' ''°'^'^'''''' ' and salt-water (Idotea) habitats, and 



a number of genera are terrestrial 

 (Porcelho, Oniscus). In these last, the head bears but a single 

 pair of antennae. 



Very great degeneracy has accompanied the acquisition of 

 the parasitic habit in a number of forms. 



Outline of Classification 



Phylum Arthropoda. — Segmented; with jointed, paired appendages; 

 chitinous exoskeleton. 



I. Class Crustacea. — Skeleton limy; two pair antennae; chiefly aquatic. 



a. Subclass Trilobita. — Exclusively marine; fossil only; two longi- 

 tudinal grooves separate body into three lobes. Triarthrus. 



b. Subclass Phyllopoda. — Most appendages leaflike. 



1. Order Branchiopoda. — Ten or more pairs trunk appendages; 

 usually with carapace. Apus, Lepidurus, Estheria, Limnadia, 

 Limnetus, Artemia, Branchinecta, Eubranchipus. 



2. Order Cladocera. — Not more than six pairs trunk appendages; 

 usually a carapace including all but head. Dapkjxia, Lcptodora 

 Polyphemus, Sida, Pleuroxus, Chydorus, Bosmina, Moina. 



c. Subclass Copepoda. — No carapace; trunk appendages biramous, 

 four or five pairs; no appendages on abdomen. 



1. Order Eucopepoda. — No compound eyes. C yclop s, Epischura, 

 Diapiomus, Ergasilus, Lernaea. 



2. Order Branchiura. — Two' compound eyes. Axgulus. 



