II. HOMOLOGIES OF CRUSTACEA. 



1. General Typical Structure of the Body. Notwithstanding the 

 great diversity of forms among Crustacea, there is in general little 

 difficulty in tracing out the typical structure through all its many 

 modifications, and distinguishing the true relations of the parts, even 

 in the most aberrant species. 



Before entering on this subject, it is important that we should ex- 

 plain what we understand to be the typical structure in Crustacea. The 

 investigations of Audouin and Milne Edwards have supplied the 

 science, we think, with correct knowledge on this point. According to 

 Edwards, the body of a Crustacean consists normally of twenty-one seg- 

 ments, fourteen belonging to the head and thorax, and seven to the 

 abdomen. In some species, as the Choristopods, seven of the first 

 fourteen pertain to the thorax, and seven to the mouth and anterior 

 part of the body or head ; but as the mouth-organs may become legs, 

 and the legs mouth-organs, by slight variations, this last-mentioned 

 division is far from general. The segments are as follows : 



f 1st segment, ophthalmic. 



I. CEPHALOTHORAX. \ 2( * se g ment > lst antennar y- 

 , , < 3d segment, 2d antennary. 



14 segments. Y J 



1 4th segment, mandibular. 



V^ 5th to 14th segments, maxillary and podal, ten pairs. 



/" 15th to 19th segments, bearing abdominal feet or appen- 



II. ABDOMEN. S dages^w pairs. 



_ , ^ 20th segment, bearing caudal abdominal appendages, 



J one pair. 



^ 21st or caudal segment, without appendages. 



The variations among species, as brought out by Audouin, depend 

 on the modifications which the normal segments may undergo by en- 

 largement, diminution, coalescence, or obsolescence, together with such 



