Mr. C. Spence Bate on the Affinity of Praniza with Anceus. 169 



The succeeding segments are as broad as the cephalon. 



The three posterior segments of the pereion are slightly nar- 

 rower, and imperfectly fused together. 



The pereiopoda bear a close general resemblance to those of 

 Praniza, though somewhat more tuberculated. 



The pleon is much narrower than the pereion, and consists of 

 six segments, each of which is furnished on each side with a pair 

 of ciliated foliaceous appendages attached to a uni-articulate 

 peduncle. The posterior segment terminates in a point. 



Upon comparison, therefore, of Praniza with Anceus, we find 

 differences as important as those which usually exist between 

 genera or even families. Hence their classification by naturalists 

 into separate genera, as Anceus and Praniza, each of which has 

 been taken as the type of a particular group or family. 



Other observers, knowing the frequent distinction that exists 

 between the forms of the different sexes of the same species, 

 have assumed that the distinction between Anceus and Praniza 

 is one of sex only. To this idea I had a considerable inclination. 



Examination of the details of both animals shows us no di- 

 stinction that is not reconcilable with this idea. The mandibles 

 resemble each other in form and position, and differ only in size 

 and strength. The labium is absent in Anceus, and developed 

 into a siphon in Praniza, — a distinction, I am informed by Pro- 

 fessor Kinahan, that was first pointed out by Mr, Haliday. But 

 that this may be only a sexual distinction, we may infer from 

 the fact that the males of parasitic Isopods differ in a similar 

 respect from their sedentary females. 



I have hitherto been inclined to believe that all Praniza were 

 females ; — that the great membranous enlargement which is 

 separated into four divisions upon the ventral surface was a 

 pouch for the development of the ova, and the homologue of the 

 pouch that is carried upon the ventral surface of the pereion in 

 all the Edriophthalmous Crustacea. 



There is certainly nothing in the young of Praniza from 

 which we could assume that an Anceus might not be deve- 

 loped. This appears still more correct when the larva has grown 

 a little, as may be seen in fig. 3. PI. VI., where the form is 

 intermediate between Praniza and Anceus, 



Recently M. Hesse has astonished us by the statement that 

 Anceus is the adult animal, and that Praniza is the young; — 

 that he has, if I understand correctly from the short notice in 

 the ' Comptes Rendus */ witnessed not only the change of the 

 former into the latter, but, moreover, the reproduction of the 

 latter from Anceus. 



* March 22, 1858, p. 568. 



