THE CRUSTACEA 



Conchostraca the body tends to be laterally compressed, and is 

 completely enclosed in a bivalved shell closely simulating that of 

 some lamellibranchiate molluscs (Fig. 17). In the Cladocera the 

 body is composed of few somites and its segmentation is more 

 or less obscured, while the bivalve shell, in most cases, covers the 

 body and limbs, leaving the head free (Fig. 18). 



ad. 



md 



l-'u:. 17. 



Estlieria obliqua (Conchostraca). A, shell of female, from the left side. B, male seen from 

 the side, after removal of left valve of shell. (After Sars.) a', antennule ; a", antenna; ml, 

 adductor muscle ; /, caudal furca ; md, mandible. On the first and second pairs of trunk- 

 linibs are seen the " claspers " of the male. 



Except in the Cladocera, the head is defined from the first trunk- 

 somite. In the Anostraca it is divided into two portions by a 

 transverse groove which crosses the dorsal surface just above the 

 mandibles. This groove is also found in the Conchostraca, and it 

 is no doubt homologous with the anterior transverse groove on 

 the dorsal surface of the carapace in the Notostraca. In the 

 Conchostraca and Cladocera the front of the head is more or less 

 produced downwards forming a rostral process, and in the Notostraca 

 it is inflected, forming a sharp semicircular anterior edge continuous 

 on either side with the lateral margins of the carapace. 



