HOMOLOGIES OF CRUSTACEA. 37 



In the Caligacea, there is a subdivision of the carapax very similar 

 to what is found in the Cyclopacea. There is sometimes a very narrow 

 first antennary segment in front, distinctly articulated with the follow- 

 ing part ; then a large segment covering the second antennas and the 

 following four pairs of organs ; next a segment bearing two pairs of 

 legs, and then two segments each with a single pair, the whole corre- 

 sponding to eleven normal segments. The last four pairs of legs are 

 very similar in form and structure to those corresponding in the Cy- 

 clopacea, and the only essential difference is, that a fifth pair (twelfth 

 segment), often present in the latter for prehension in coition, is not 

 found in the Caligacea. 



The examples referred to, are sufficient to exhibit the varieties of 

 composition in the carapax or shell of Crustacea of different tribes or 

 families; and without pursuing the subject farther, we annex a table 

 showing the normal relations of the segments and members for the 

 predominant forms. In this table the normal segments are numbered 

 in the first column with Roman numerals. The fact that the appen- 

 dages of a segment are obsolete, is indicated by a zero ; and that both 

 a segment and its appendages are obsolete, by two zeros. 



10 



