444 Prof. K. Grobben on the Genealogy and 



compressed, while together with the head it is completely 

 covered by the ample bivalve shell, which is closed by a 

 muscle. The number of the segments of the body is larger 

 than in Branchijms (amounting to as many as 28), and the 

 elongate lamelliform appendages appear on all the free thoracic 

 segments, gradually diminishing in size towards the rear. 

 The end of the abdomen exhibits a very peculiar development 

 and is seen to be bent towards the ventral surface and cleft 

 into two lamella, which are armed at the end with hook- 

 shaped furcal branches. A decidedly ancient character is to 

 be observed in the second antenna, which has retained the 

 shape of a swimming-foot. The heart remains still shorter 

 than in Ajjus, and merely extends through the foremost 

 portion of the body. The two compound eyes are in complete 

 contact one with another in the median line ; at the same 

 time, just as in the case of Apus, they are sunken and over- 

 grown by a reduplicature of the skin. While, however, in 

 the case of Apus the eyes are driven to the dorsal side, in 

 consequence of the lateral extension of the carapace and the 

 inclusion of the head in its prolongation, we find them in 

 Estheria enclosed in the narrow head, above and beyond which 

 lie the valves of the shell. The peculiar position of the two 

 eyes, as well as their convergence in the median plane, is 

 occasioned by the formation of the shell, as has already been 

 explained by Dohrn *. In consequence of the inclusion of 

 the body between the valves of the shell and the lateral 

 compression connected therewith the original eye-stalks 

 degenerated and the eyes were pressed together in the 

 median line. The opacity of the shell favoured this process. 

 The overgrowth of the fused eyes by the skin may have 

 developed as a protection for the visual organ in connexion 

 with the burrowing mode of life of Estheria, just as the same 

 cause probably cooperated in the case of Apus also. It is 

 probable that the overgrowth and approximation of the eyes 

 in Apus and Estheria arose independently in the two groups. 



As a secondary character which is common to all three 

 Euphyllopod types must be mentioned the degeneration of 

 the mandibular palp and the reduction of both pairs of 

 maxillse. 



The peculiar development of the furca and the relative 

 heaviness of the valves of the shell are a sufficient indication 

 that Estheria is a form which is well adapted for motion on 

 firm ground, just as moreover, as a matter of fact, this animal 



* A. Dohrn, " Gescbichte des Krebstammes," Jenaische Zeitschrift fiir 

 Medicin imd Naturwissenschaften, Bd. vi., 1871, p. 149. 



