106 ANIMAL FORMS 
The marine isopods occur in the sand, under rocks, and 
in the seaweeds; many are parasitic upon fishes; and the ter- 
restrial forms (Fig. 63) are very common objects under old 
Fic. 64.—Amphipods or sand-fleas (Gammarus, upper species, and Caprella). 
logs and in cellars, where they live chiefly on vegetable mat- 
ter. In the sand-fleas the body is compressed from side to 
side, and while the thorax shows distinct segments, the legs 
are frequently dissimilar, and some may bear pincers. One 
of their most distinctive marks concerns the last three ab- 
dominal appendages, which are usually modified for leaping. 
The sand-fleas (Fig. 64) are familiar objects to any one 
who has collected along the beach and has turned over the 
cast-up seaweeds, while numbers of small species often oc- 
cur among the plants in our fresh-water ponds. Some most 
curious and highly modified forms, whose general appear- 
ance is shown in the lower part of Fig. 64, occur among 
