THE JOINTED-FOOT ANIMALS 147 



brightest sunshine and at night. In feeding it seems to prefer 

 plants, living upon very small green algse which grow on the 

 moist sand. The male picks up these algse with its small 

 cheliped and passes them to its mouth, or collects them in 

 pellets to carry into its hole in the same way that it carries 

 pellets of sand out of it. 



The Sow-Bug. The sow-bug, and a related species called 

 the pill-bug because of its habit of rolling up into a ball, are 

 found under stones, boards, logs, and in other dark, moist 

 places. They live on vegetable matter. The flattened con- 

 dition of the body reminds one of the cock- 

 roach and the cricket, which show the same 

 adjustment of the form of the body to the 

 necessities of their life. The species repre- 

 sented (Fig. 73) belongs to the genus Onis'cus. 



The body has twenty somites, as have the 

 crayfish and all the forms described so far in 

 this chapter; five of these, with one thoracic 

 somite, are fused in the head-region ; seven 

 thoracic somites are free-moving, and of the 

 seven abdominal somites all are free-moving 

 except the last two. A pair of short, jointed ' ^ w 



antennae and a pair of compound eyes with- 

 out stalks, i.e. sessile eyes (Lat. sedere, to sit), are the most 

 noticeable prgans of the head. They have a second pair of 

 antennae, which are rudimentary. The mouth-parts are small 

 and adapted to feeding on plant-food. The breathing organs 

 are gills, protected by flat, plate-like structures on the under 

 surface of the abdomen. The base of the legs of the female 

 bears other small plates, which, with the under surface of the 

 body, form a brood-pouch in which the eggs are carried and 

 the young developed. 



Caprella. Probably one of the strangest looking free 

 forms to be found in the sea is the little brown Caprel'la 



