^S PHYLUM ARTHROPODA 



Class : Crustacea. Division : Malacostraca. Order : Amphipoda 



A FRESH-WATER SHRIMP (GAMMARUS) OR A 

 SAND FLEA {TALORCHESTIA) 



The fresh- water 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 fea- 

 ture is extremely widespread among the smaller aquatic ani- 

 mals. Can you explain what is the advantage to a small aquatic 

 animal of being 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 apparent head is 

 composed of six fused somites, five being cephalic and one tho- 

 racic. There are thus seven free thoracic segments. Note the 

 broad movable plates, the epimeral plates, which extend down- 

 ward from the ventral side of certain of the thoracic segments ; 

 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. 



Study the appendages, beginning with the free thoracic ones. 

 With fine needles separate the legs and observe the gills attached 

 to the posterior borders. How many bear gills? In the female 

 observe the brood sac when it is present ; it is formed by plate- 

 like projections of the inner side of certain thoracic feet. In the 

 abdomen observe the biramous appendages ; they bear no gills. 

 The cephalic appendages are those typical of Crustacea. In front 

 of the mouth is a median lip called the labium. Then come the 

 mandibles and two pairs of maxillae. The pair of appendages, the 

 maxillipeds, belonging to the first thoracic somite (which is fused 

 with the cephalic somites) form a kind of lower lip. 



Exercise. Draw an outline of the side view of the animal on a large 

 scale. Number the thoracic and the abdominal segments. 



