SECT, xv CLASSIFICATION OF CRUSTACEA 271 



THE ESTHERID^E. 



This is the only other group of the Phyllopoda 

 about which we have a few words to say. The forma- 

 tion of the bivalve shell of these animals has already 

 been noticed. The question is, How can a perfect 

 bivalve shell, enclosing the whole body, head and all, 

 be deduced from the folding down of the lateral 



FIG. 61. Limnetis brachyurus, 9 O. F. Miiller (from Bronn's Kiassen und 

 Ordnungeii), to show the position of the head as transition stage between the 

 Cladocera and the Estheridae (Fig. 62). 



halves of a dorsal shield ? It fortunately happens 

 that we have a series of forms which make the point 

 quite clear. 



In the Cladocera, we have the shell folded down 

 against the sides of the animal, leaving the head quite 

 distinctly marked off (Fig. 60). In Limnetis we find 

 the lateral folds of the shell extending more ante- 

 riorly so as partially to enclose the head, the change 



