104 ZOOLOGY 



after and prized on account of their supposed medicinal 

 qualities. The function of these gastroliths or " crab's- 

 eyes " was for a long time rather obscure. It is now 

 believed that during the time of absorption of lime from 

 the shell, previous to moulting, the blood becomes strongly 

 impregnated with lime. If all the lime that must be re- 

 moved were to remain in the blood, it would probably be 

 fatal to the animal ; hence it is taken up by secreting cells 

 located in the wall of the stomach, and there deposited. 

 After the old skin is cast, the gastrolith is soon absorbed, 



FIG. 96. Palsemonetes vulgaris, a common shrimp. Nat. siza. 

 Photo, by W. H. C. P. 



probably to aid in strengthening the new shell. Bits of 

 water-worn shells, entire gastropod shells, parts of lobster 

 coverings, spines of sea-urchins, etc., have been found in 

 the stomachs of lobsters and crayfish, which likewise would 

 probably have been dissolved and used in hardening the 

 shell. 



Shrimps and prawns 1 belong to a thin-skinned, long-tailed 

 family of Crustacea. 2 They are extremely common in bays 



1 These are common names of small Crustacea applied chiefly to the 

 decapod family Carididse,* although also applied to certain Schizopods. 

 The term shrimp is applied to the smaller species, and prawns to the 

 larger. 2 Fig> QQ. 



* Kapts, a small marine crustacean. 



