48 INVERTEBRATE ZOOLOGY 
CRUSTACEA 
A TYPICAL AMPHIPOD. 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 antenne. 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 1. Draw an outline of the side view of the animal on 
a large scale. Number the thoracic and the abdominal 
segments. 
