36 CRUSTACEA. 



In Branchipus, the same structure in this respect exists as in the 

 Choristopods. 



In Cypridina, of the Cypris group, the shell evidently corresponds 

 to the second antennary or inandibular segment, or both, since it is 

 in the neighbourhood of these parts that it is connected with the 

 body. There is a dorsal union in the vicinity of the second antennae, 

 and a large transverse muscle either side, for closing the shell, that 

 passes out from near the base of the mandibles and maxillae. It 

 seems probable, from facts observed in the Daphnia and Cyclops groups, 

 that both the second antennary and mandibular segments are included, 

 and the latter, perhaps, most largely. 



In Apus there is the same dorsal union as in Cypridina. 



In the Daphnia group, as already observed, there is a distinct seg- 

 ment of the body to each of the natatory legs, and therefore the 

 buckler or shell must pertain to an anterior segment, and either to a 

 maxillary, a mandibular, or a second antennary segment. Between 

 the cephalic part bearing the antennae (the second antennae alone 

 are present) and the rest of the carapax, there is often a suture, sepa- 

 rating the shell into two distinct parts. The anterior part is evidently 

 the second antennary segment; the posterior must belong therefore to 

 the mandibular or maxillary segment, and probably to the former. 



In the Cyclopacea, there is in Cyclops a single segment covering the 

 cephalothorax to the ninth normal segment, and the ninth, tenth, 

 eleventh, and twelfth segments (the following being wanting and the 

 twelfth often so), are each distinct. From the close relations of these 

 species to the Schizopods, we might infer that the shell of the large 

 anterior segment belonged mainly to the second antennary segment. 

 But a suture and constriction in species of the genus Pontia separates 

 a head segment which is properly the antennary. The rest of the 

 large segment, from analogy with the Daphnidae, might then be attri- 

 buted to the mandibular ring. But in many species of the same 

 genus, there is another suture or articulation intersecting this segment 

 near its middle, between the sixth and seventh normal rings, the first 

 of the two bearing the mandibles, a pair of maxillae, and a pair of 

 maxillipeds, and the second, two pairs of legs. There is in this case 

 a very different relation of the shell, approaching that in the Choris- 

 topods. It appears therefore to be a fact, that in some cases when the 

 shell grows with an attachment along the whole thorax, the annula- 

 tions, corresponding to the members below, may reach the surface. 



