Classification of the Crustacea Malacostraca. 153 



Peracarida the "claw " is coalesced with the segment which 

 carries it, the suture-line between the two disappearing and 

 the place of junction being indicated, if at all, only by the 

 insertion of a minute seta, and it is not impossible that such 

 evidence of the existence of a " claw " may yet be found in 

 the terminal segment of the decapod leg. In the absence of 

 any definite proof that the fourth segment of the leg in 

 the Eucarida represents two fused segments, it seems better 

 to assume for the present that the segments of the legs are 

 serially comparable in the two groups. 



Dr. Hansen includes among the characters of the Pera- 

 carida the presence of tubular processes for the orifices of 

 the vasa deferentia, which are stated to be absent in the 

 Eucarida. It is true that such processes are present in tiie 

 majority of the Peracarida, though they are sometimes much 

 reduced and may perhaps be altogether wanting in some 

 cases. They are absent in the Euphausiacea and in the 

 lower Decapoda, but in some Paguridea and in the Brachyura 

 the vasa deferent i a terminate in tubular processes wliich are 

 often of considerable length. 



The possession of spermatophores is another character on 

 which it seems unsafe to rely as distinguishing the Euphau- 

 siacea and Decapoda from the other orders of Malacostraca. 

 It certainly constitutes an important difference between the 

 Euphausiacea and the Mysidacea, but it can hardly be ex- 

 tended without qualification to some of the other groups. 

 Prof. Gilson applies the term " spermatophores " to the 

 aggregations of spermatozoa found in certain Isopoda *, but 

 not to the sperm- masses of the Macrura f. The distinction 

 which Prof. Giard \ makes (in Insects) between spermato- 

 phores and " spermotagmata/' according to the presence or 

 absence of a definite investing membrane, appears to be hard 

 to recognize among Crustacea and to have little systematic 

 importance §. On the other hand, the form of the spermatozoa 

 appears to afford constant and important characters differen- 

 tiating the two groups. 



* " £tude coniparee de la spermatogenese cliez lea Artbropodes," La 

 Cellule, i. (1884) p. 158. 



t Op. cit. ii. (1887) p. 187. 



X " rfur la spermatogenese des Dipteres du genre Sciara,^' C. R. Acad. 

 Sci. cxxxiv. (1902) p. 1124. 



§ Prof. McMurricli describes (" Embryology of the Isopod Crustacea," 

 Journ. Morph. xi. 1895, p. 67) a very defiuite spermatophore in the Isopod 

 Jeera in connexion with the process of " hypodermic impregnation " 

 -which he believes to occur in that genus; but his account is not very 

 detailed, and the phenomena which he describes are so remarkable that 

 further investigation is much to be desired. 



