41 ti AETHBOPOPA. EXTOHOSTBACA. 



of Crustacean orders, for the most part fossil and belonging to tlie 

 oldest formations, which present in their development no certain 

 trace of the Nauplius form so characteristic of the true Crustacea, 

 and are in all probability related to the Arachnoidea. These orders, 

 which may be grouped together as the Gigantostraca, are the 

 Jferostomata and Xiphosura, to which the Trilobitn are possibly allied. 



1 . ENTOMOSTRAC A. 

 Order 1. PIIYLLOPODA.* 



Crustacea with elongated and often distinctly segmented body ; 

 usually with ajlat, shield-like carapace, or laterally compressed bivalve 

 shell, formed by a reduplicature of the, skin. There are, at least, four 

 pairs of leaf -like lobed swimming feet. 



The animals belonging to this order differ very considerably in 

 form and size, in the number of their segments and appendages, as 

 well as in their internal structure. They all, however, agree in 

 the structure of their lobed, leaf -like feet. In their form, internal 

 organization and development they appear to be the most primitive 

 of Crustacea, and may be regarded as the least modified descendants 

 of ancient types. 



The body is either cylindrical, elongated and clearly segmented, 

 without free reduplicature of the skin, e.g. Branchipus (fig. 333), 

 or it may be covered by a broad and flattened shield, which only 

 allows the posterior part of the body to project uncovered, e.g. Apus. 

 In other cases the body is laterally compressed and is enclosed by a 

 bivalve shell, from which the anterior part of the head projects 

 (Cladocera) ; or finally the laterally compressed body is completely 

 covered by a bivalve shell (Estheridce). Sometimes the head is 

 more sharply distinct, while the thorax and abdomen are not so 

 clearly distinguishable from each other. As a rule, the posterior 

 segments only are without appendages. The hind end of the 

 abdomen is very often curved ventralwards and forwards, and 

 bears two rows of posteriorly directed claws, the two last of which 

 arise at the point of the caudal appendage, and are by far the 



* Besides the works of 0. Fr. Miiller, Jurine, M. Edwards, Dana, compare 

 Zaddach, " De Apodis cancriformis anatome et historia evolutionis," Bonnse, 

 1S41. E. Grube, '' Bemerkungen liber die Phyilopoden," Arclnvfiir NatwrgetoTt, 

 1853 and 1855. Fr. Leydig, " Monographic der Daphniden," Tiibingcn, 1860. 



