

4Q CRUSTACEA. 



Umulw.-In Limulus, the body consists of three segments, and they 

 may he compared to the segments in Caligus. The anterior seg- 

 ment bears six pairs of members; the first appears to correspond to 

 the second pair of antenna? (or third normal segment), the second, 

 tliird, fourth, fifth, and sixth, to the mandibles and the four following 

 pairs of members (or the fourth to the eighth normal segments in- 

 clusive ) . In the Caligus, the last pair here referred to is natatory, and 

 the carapax is divided just anterior to it, instead of posterior. 



The second segment of the body, which we consider as a continua- 

 tion of the cephalothorax, and not abdominal, bears six pairs of 

 foliaceous organs, analogous to the foliaceous appendages of the pos- 

 terior part of the thorax, in certain Caligidse, in some of which, one or 

 two pairs of legs are combined into a broad thin plate, like an apron. 

 These six pairs make up exactly the normal number of cephalo- 

 thoracic segments,— fourteen. It is an objection to viewing this seg- 

 ment as abdominal, that in no Entomostracan is the abdomen pro- 

 vided with branchial appendages. Moreover, the close relation to the 

 Caligida\ — the resemblance as regards the general form and subdivi- 

 sion of the shell, supposing the two segments both cephalothoracic, — 

 and the near resemblance between the foliaceous appendages and the 

 oephalothoracic appendages, in certain Caligi as well as in Apus 

 and the allied, are believed to be good reasons for adopting the opinion 

 which we have here brought forward. 



The abdomen, according to this view, is confined to the last or third. 

 segment. 



7. Homologies of the Phyllopoda. — The Phyllopoda, in which the 

 number of segments exceeds the normal number, offer a difficult 

 problem to science, viz., the determination of the normal relations of 

 the appendages. In Branchipus, the number of segments is twenty- 

 two, of which nine belong to the abdomen, eleven to the body posterior 

 to the second pair of maxillse ; seven being the normal number for the 

 former, and eight for the latter. In Limnadia, there are eighteen or 

 twenty-seven pairs of thoracic members following a pair of maxillse 

 and mandibles. In Apus, there is a pair of mandibles, then two of 

 maxillae, then a large series of legs, all of which are more or less folia- 

 ceous excepting the anterior. In Nebalia, the abnormal character is 

 the same, although the members are not as much multiplied. 



