108 M. L. Plate on Ectoparasitic Rotatoria 



Ductifera which are sessile when adult [Lacinularia^ Meli- 

 certOy and their allies). Thej have further a horseshoe- 

 shaped vitelligene, each limb of which bears an ovary of 

 the usual construction at its posterior end, that of the left side 

 of the body being feebly, and that of the right side more 

 strongly developed, and the latter alone seeming to function. 

 This doubling of the ovary renders it very probable that the 

 two limbs of the viteUigene were also originally separate, and 

 only became united subsequently. A vitelligene so con- 

 structed, indeed, still occurs in some species of the genus 

 Asplanchna, which in other respects differs greatly from 

 Pterodina, Here the limbs of the organ are so long in pro- 

 portion to the part uniting them, that one is involuntarily 

 driven to the assumption of their original duplex nature, 

 although the ovary, which is placed at the apical point of the 

 two limbs, is unpaired. The two genera just named, there- 

 fore, as regards the structure of the female sexual organs, 

 facilitate the passage from the two older families of the Philo- 

 din?ea3 and Seisonidte to the more modified younger family of 

 the Ductifera. 



This view as to the phylogenetic development of the 

 natural groups of the Rotatoria meets only with one difficulty. 

 The masticatory apparatus of the Seisonidee closely approaches 

 the typical structure of this organ in the Ductifera, but differs 

 considerably from that of the Philodina3a3. According to 

 Gosse*^ one can distinguish in the mastax of the majority of 

 the Rotatoria a central " incus " composed of two " rami " 

 and two lateral " mallei," parts which may also be recognized 

 at once in Paraseison. In the Philodinasaj, on the contrary, 

 these ossicles are fused together into two ribbed masticatory 

 plates, which also reappear in exactly similar development in 

 many Melicertidaj. These facts admit two hypotheses for 

 their explanation ; either the biting- organs of the Arclm-otatov 

 were like those now presented by the Philodin£ea3, and deve- 

 loped themselves therefrom, independently of each other, in 

 two different sections ; or, and I regard this as more probable, 

 they showed from the beginning the structure still existing in 

 the majority of the Rotatoria, from which, then, the two mas- 

 ticatory plates which we now find in the Philodinffite and 

 some Ductifera were produced by the growing togetlier of 

 the chitinous bands belonging to each half of the body. 



In conclusion, a condensed summary of the most important 

 anatomical points for the distinction of known genera and 

 species of the family Seisonidos may here follow : as a matter 

 of course it represents only the present standpoint of our 



* Gosse, "Manducatory Org'ans of Rotifera/'in Pliil. Trans. 1856, 



