Classification of the Crustacea. 443 



ancestral form. The lateral margins of the shell in Apus are 

 directly continuous with the anterior border of the head, a 

 peculiarity to whicli we shall have to revert later on. Diver- 

 gence from the original ancestral form is seen in the special 

 development of the thoracic appendages. As opposed to 

 Branch'pus^ in which the appendage is expanded like a leaf, 

 we tind in Apus a narrow elongated axis and an elongated 

 stiff form of endites, the most distal of which (the sixth) 

 corresponds to tlie endopodite*. It is true that this elongated 

 form is in the first instance assumed only by the anterior 

 appendages, while the posterior ones are very broad. Yet in 

 my opinion the anterior limbs of Apus (although not exactly 

 the two first, which have undergone further modification) 

 exhibit the more primitive form with reference to the shape of 

 the larval limbs, and also with regard to the form of the 

 appendages which must be assumed for the ancestral types. 



The furcal appendages in Apus are elongated and deve- 

 loped into the shape of filaments. As a secondary character 

 must be regarded the total loss or the far-reaching degenera- 

 tion of the second antenna, the original function of which as 

 a swimming-foot has been taken over by the first thoracic 

 appendage, which is furnished with long flagelliform processes. 

 The heart does not extend, as in Branc/n'pus, throughout the 

 whole of the segments of the body, but is confined to the 

 anterior half of the trunk, a phenomenon which, when con- 

 trasted with the primitive condition met with in Branchipus^ 

 must be regarded as of a secondary nature. In a similar 

 fashion is to be interpreted the displacement of the compound 

 eye in Apus. The two eyes are not situated upon stalks, but, 

 as I have previously shown f, are sunken and covered by 

 a reduplicature of the skin ; at the same time they are closely 

 approximated to the median line. 



A type which in general appearance diverges very widely 

 from Branchipus as well as from Apus is constituted by 

 Esiheria. In this case the body is thickset and laterally 



* I am unable to asseut to the interpretation g-iven by Ray Lankester 

 (" Observations and Eeflections on the Appenda^^es and on the Nervous 

 System of ^;;ms ca«C7-{/o/-ww'A'," Quart. Journ. Micr. Sci. vol. xxi., 1881, 

 p. 363) of the sixth endite as the exopodite, and of the fifth as the endo- 

 podite, since the facts of embryology go to show that the sixth endite 

 corresponds to the endopodite and tlie flabellum to the exopodite. — Cf. 

 C. Claus, " Zur Kenntniss des Baues und der Entwicklung von Branchipus 

 stagnalis und Apus cancriformis,^'' Abhandlungeu der koaigl. Gesellschaft 

 der Wissenschaften zu Gbttingen, xviii. Bd., 1873, p. 20. 



t Cf. C. Grobben, "Die Entwicklungsgeschichte der Moina rcctiros- 

 ti'is. Zugleich ein Beitrag zur Kenntniss der Anatomic der Phyllopoden," 

 Arbeiten des zoolog. Institutes zu Wien, Bd. ii., 1879, pp. 51 et seq. 



