48 INVERTEBRATE ZOOLOGY 



CRUSTACEA 



A TYPICAL AMPHIP0D. A FRESHWATER SHRIMP (Gammarus) 

 OR A SAND-FLEA (Ta/orchestia) 



The freshwater shrimp is common in many places in pools 

 and streams, and may be easily caught with a fine net ; the 

 sand-flea is a marine animal and is extremely common along all 

 of our shores. 



Notice the compressed and translucent body ; this latter 

 feature is extremely wide-spread among the smaller aquatic 

 animals. Can you explain what is the advantage to a small 

 aquatic animal to be translucent or transparent ? Note the two 

 pairs of long antennae. In common with all the higher Crustacea, 

 the body is composed of twenty somites, of which five are 

 cephalic, eight thoracic, and seven abdominal. Like the isopod, 

 the animal has no carapace, the eyes are sessile, and the appar- 

 ent head is composed of six fused somites, five being cephalic 

 and one thoracic. There are thus seven free thoracic segments. 

 Note the broad movable plates, the epimeral plates, which depend 

 from the ventral side of certain of the thoracic segments, 

 extending the lateral surface of the body ventrally; note the 

 differences in form between the thoracic appendages. The 

 abdomen is composed of six free segments, the sixth and 

 seventh somites being fused. Count them. The first three 

 pairs of abdominal legs are swimming legs, the last three are 

 jumping legs. 



Exercise l. Draw an outline of the side view of the animal on 

 a large scale. Number the thoracic and the abdominal 

 segments. 



