CRUSTACEA. 



47 



in an area (Fig. 6, k) marked by a series of concen- 

 tric lines caused by the excess of chitinous matter, and 

 at various parts of the internal rings of the thorax, at 

 the bases of the legs, and along the sutures of the 

 carapace. When this absorption proceeds far enough. 

 the carapace splits under the pressure, otherwise it re- 

 mains whole, as in the case above described. 



EDRIOPTHALMIA. 



Jsopods. 



It has been thought best to treat only of those forms 

 which may be easily obtained and maybe most directly 

 compared with the lobster in certain logical sequence 

 in this part of the Guide, and defer even the brief men- 

 tion which is possible of the lowest forms of the Crus- 

 tacea until this was finished. The Isopods are found 

 on our coast between tide-marks, 

 and are related to the common 

 •• sow bug," Oniscus (Fig. 19), so hd\ 

 often seen in cellars, and common 

 also beneath stones, boards, etc., 

 and other damp places. A species 

 ofAsellus also occurs in our fresh 

 waters which is like the Oniscus. 

 and can be used as a type. They 

 are well represented by the [do- 

 teea (Fig. 20). which is very com- 

 mon in eel-grass, beyond low-wa- 

 ter mark. Its body is flattened, and the abdomen and 

 thorax are continuous, and the freely movable rings 

 of the latter may be easily counted. The carapace is 



Oniscus (after Morse). 



