Classification of the Crustacea. 445 



readily burrows in mud. In this respect among the three 

 typical Euphyllopods it differs most widely from Branchipus, 

 which appears to be the best swimmer of the group. Apus 

 occupies about the middle position ; it is a good swimmer, 

 but is also fond of remaining at the bottom, where it digs up 

 the mud with its shield "^. 



The Cladocera. 



On considering the structure of the Cladocera we shall be 

 struck by their great agreement with Estheria, while a com- 

 parison with Apus or Branchipus cannot be sustained in the 

 same manner. A full idea of the far-reaching similarity 

 between the two first-mentioned forms is acquired, how^ever, 

 when we select for comparison a young Esthena at a stage in 

 which some six thoracic feet are present. A stage such as this 

 was described by Joly f, by Ficker|, and likewise by Glaus §, 

 while by the latter it was also employed for the purpose of a 

 searching comparison with the Cladocera, in the sense of the 

 closest original relationship. An appeal to the existing 

 statements on the subject will here suffice, and I will merely 

 refer to the most important points of agreement. 



In the Cladocera, just as in the case of Estheria, the body 

 is laterally compressed. The shell is bivalve and covers the 

 entire body, with the exception, however, of the head, which 

 remains uncovered. The furcal end of the abdomen exhibits 

 the development which is foand only in the Estheria-type 

 among the Euphyllopoda ; as in the case of Estheria, it is 

 bent towards the ventral surface and is furnished at its extre- 

 mity with backwardly-directed hooks. The development of 

 the second antenna as a swimming-organ is common to both 

 the Cladocera and Esther-ia. Similarly the absence of the 

 mandibular palp as well as the reduction of the two pairs of 

 maxilla3, of which the second is entirely wanting in the 

 Cladocera, has been inherited from the Euphyllopoda, in this 

 case from Estheria. The shape of the thoracic feet can like- 

 wise be derived from that of those of Estheria, and the more 



* For the statements as to these biological conditions I am indebted to 

 Prof. Brauer. Cf. also Bronn's " Classen und Ordnungen des Thier- 

 reiches," Arthropoda, bearbeitet von A. Gerstaecker. I. Crustacea. Erste 

 Halfte, pp. 1049 et seq. 



t N. Joly, " Recherches zoologiques, anatomiques et physiologiques 

 sur VIsaura cydaduides^^ Ann. Sc. Nat. 2^ ser. t. xvii., 1842, p. 325. 



X G. Ficker, " Zur Kenntniss der Entwicklung von Estheria ticinensis" 

 Sitzungsber. k. Akad. Wiss. Wieu, math.-naturw. Classe, Bd. 74, 1876. 



§ C. Glaus, ' Untersuchungen zur Erforschung der genealogischon 

 Gruudlage des Crustaceensystems,' Wien, 1876, p. 101. 



