34 CRUSTACEA. 



The anterior part of the longitudinal sutures and the transverse 

 dorsal suture, taken together, constitute, therefore, the analogue of the 

 transverse suture in Astacus. The anterior segment thus cut off, is 

 the true first antennary ; it is the only part which reaches this pair of 

 organs. The posterior segment consists of a dorsal piece and two 

 lateral, and cannot therefore be an epimeral to the anterior; it is 

 rather a distinct segment, with its own epimerals separate. The 

 dorsal segment of these three, is either narrow linear, as in Thalassinea 

 (and also in Pagurus), or broad, as in Gebia. In Astacus, there are 

 traces of the same division of the posterior part of the carapax into 

 three parts, a dorsal and two lateral, and the dorsal piece is very nar- 

 row in some species (as A. affinis), and broad in others. There is a close 

 correspondence with the structure exemplified in the Thalassinidea. 

 The posterior part of the carapax must, therefore, be a separate seg- 

 ment, and is mainly if not wholly, the mandibular segment. We say 

 mainly, for in Thalassina there is some reason for believing the poste- 

 rior dorsal segment and the lateral pertaining to it, to include also 

 two or three segments, more posterior, as there are transverse sutures 

 indicating their limits. This point, and others bearing on this sub- 

 ject, are illustrated in our remarks on the group Thalassinidea. 



A dissimilarity between related forms, like that described, is no un- 

 usual fact among Crustacea. In Apus and Cypris, the carapax is evi- 

 dently either mandibular or second antennary, as in the Brachyura; 

 for the body is attached to the shell only by its anterior portion. In 

 Daphnia, closely related to Cypris, as explained beyond, there is a dis- 

 tinct cephalic suture, so that only a small anterior part of the carapax 

 is second antennary, and all the rest is probably mandibular. Pass now 

 to one of the Cyclopidaa. In these elongate forms, not far removed from 

 the Daphniae, the shell of the cephalothorax, instead of pertaining to one 

 or two segments, has distinct articulations behind, making, it may be, 

 four segments in all; one quite oblong anteriorly, and the other three 

 or four posteriorly. The anterior one, we might, perhaps, infer from 

 analogy, to pertain to the mandibular or second antennary; but there 

 is sometimes a cephalic segment, as in Daphnia, bearing only the 

 second and first pair of antennae ; and there is also in other species still 

 another suture posterior to the cephalic. Thus, the subdivisions of 

 the shell of the cephalothorax are dependent on its connexion with 

 the body. It may, as in Daphnia, belong to the anterior segments 

 alone ; or, as in Cyclops, to a series of segments. So, in the Eubran- 



