154 M. A. Giard on the Genus Entoniscus. 



The presence of a nauplius-eye very distinctly formed in 

 the embryo of E. Cavolinii appears to me to be of some im- 

 portance as a trace of the nauplius phase in the embryogeny 

 of the Isopods. Hitherto we had no factual argument to 

 appeal to in order to connect the Isopods with the original 

 form common to all the Crustacea. The opinion of Fritz 

 Muller, who regards the embryonic membrane of Ligia or 

 Ouiscus as representing the nauplian skin, seems to me desti- 

 tute of foundation. In all the groups in which embryonic 

 membranes exist, these membranes are superadded in certain 

 forms as protective organs of the typical embryo without 

 modifying its essential characters. They are in general ex- 

 odermic folds performing the part of an amnios. This is what 

 takes place, for example, in insects, where these membranes 

 may be formed in various fashions, and have no real morpho- 

 logical significance from the point of view of comparative 

 embryogeny. These membranes are most frequently deter- 

 mined by physiological reasons, and may disappear or be re- 

 tained in very nearly allied types. 



The presence of the eye, so characteristic of the nauplius, 

 seems to me, on the contrary, a mark of great value for the 

 phylogeny of the Arthrostraca. 



Each of the feet of the first five thoracic pairs terminates in 

 a prehensile hand (fig. 8), of which the penultimate joint is 

 oval, and bears two denticles on the side which is turned to- 

 wards the opposable claw. 



The sixth pair of thoracic feet, so exceedingly characteristic 

 for the distinction of the species of the genus Entoniscus, does 

 not at all resemble the same part in the types hitherto de- 

 scribed. It consists of five joints ; that which corresponds to 

 the hand of the other pairs is more elongated (fig. 6, a), and 

 terminates, on its inner margin, in a small curved fixed tooth ; 

 its outer border is produced into a straight rod, as long as the 

 joint which supports it, and furnished at its extremity with a 

 tuft of rigid setas. 



Here, then, we find a remarkable confirmation of the law 

 demonstrated by Darwin and Fritz Muller : — When, in a group 

 of animals, an organ presents an exceptional development, 

 this organ is at the same time subject to great variability in 

 the various species of the group. 



It is probable that this sixth pair of feet aid the embryo to 

 make its way into the interior of the crab in which it is to 

 undergo its retrograde metamorphosis. The variations which 

 it presents in the different species of Entoniscus are conse- 

 quently in relation to the peculiar conformation of the branchial 

 cavity of the animal infested. 



