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 



FIG. 64. Amphipods or sand-fleas (Gammarus, upper species, and Capretta). 



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 



