HOMOLOGIES OF CRUSTACEA. 23 



rapid in development ; next, rudiments of the eyes are seen ; about 

 this time, the maxillae and maxillipeds are developed in succession, 

 first the three pairs of maxillae, then the following two pairs ; and, as 

 these continue enlarging, the feet finally become apparent, the ante- 

 rior pairs being earliest. The succession is thus in a line, either way 

 from the mandibles. The mandibles at the centre are often the 

 shortest of the organs, and in the Decapoda, the size increases from 

 these forward and backward, becoming largest in the posterior series, 

 usually in the sixth pair following the mandibles (the first pair of 

 feet in the Decapods,) and in the anterior series, in the posterior or 

 anterior antennas, usually the former. Notwithstanding the diversity 

 of results, the general fact of progression from a single centre, holds 

 true for the cephalothorax, and strongly confirms the view, that in 

 Crustacea this portion of the body is a unit of itself. 



While, therefore, fourteen is the whole number of successive parts or 

 pairs of parts in the cephalothorax, we cannot properly divide them, 

 and attribute a particular number to the head and the rest to the 

 thorax. 



3. Homologies oftJie Carapax among Crustacea. In the study of the 

 hoinologies of Crustacea, the true relations of the carapax to the other 

 parts must first be correctly understood; and here lies one of the prin- 

 cipal difficulties in this department of investigation. Milne Edwards 

 has ably met the subject and arrived at the conclusion, that in the 

 Decapods, the shell properly pertains to the third or fourth of the 

 normal segments of the body, that is, to the second antennary or the 

 mandibular segment. The argument on this point, drawn from cer- 

 tain Stomatopods, as the Squillidse, in which all the rings or segments 

 are distinct and may be counted, excepting the third and fourth, 

 which appear to be blended, is satisfactory as to the main point. The 

 same structure is found also in some of the Entomostraca ; yet it is 

 not universal among these species, as is seen in the Cyclopacea, Cali- 

 gacea, and others ; whose relations will be considered beyond. But 

 the question as to which of these two segments, the second antennary or 

 mandibular, the body of the carapax belongs, has not, hitherto, been 

 decided. 



Some facts have been observed by the author which lead to a con- 

 clusion on this point. It is evident, when the carapax of a crab is 

 separated from the body, that it is an anterior segment prolonged 

 far posteriorly ; for its sides are free, and only at the anterior extre- 



