106 M. L. Plate on Ectoparasitic Rotatoria 



structure. In all other peculiarities of organization, however, 

 there prevails a complete agreement with the above-described 

 species of Paraseison ; there is no intestine, the water- vascular 

 system forks in the trunk into a broad and a narrow branch, 

 in the hinder part of the head there are only two pyriform 

 glands, which open into the oesophagus at the commencement 

 of the neck, and in the so-called ductus ejaculatorius^ the wall 

 of which shows no musculature, there are a number of the 

 flask-shaped structures which liave been regarded as sperma- 

 tophores. The masticatory apparatus and the dimensions of 

 different divisions of the body are also as in P. asjjlanchnus. 



5. On the Systematic Position of the SeisonidcBy and the 

 Natural Oroui^s of the Rotatoria. 



The new genus Paraseison described in this memoir cer- 

 tainly differs from the previously known genus Seison in 

 many particulars ; but these are of so slight a nature that no 

 doubt can exist as to the close relationship of the two forms. 

 A fresh investigation of the ectoparasites occurring on Nehalice 

 near Trieste will probably also furnish evidence of some things 

 hitherto found only in Paraseison^ such as the dorsal feeler, a 

 greater number of flicker-organs, and a ventral branch of 

 the part of the water-vascular system situated in the trunk. 

 As to the position which the genus Saccobdella occupies with 

 regard to these two genera further investigations must decide, as 

 our knowledge of this animal-form is at present very imperfect, 

 and we can only unite it provisionally and hypothetically with 

 Seison and Pai-aseison, in the family Seisonidre. The relations 

 of this group to the other Eotatoria will be elucidated in what 

 follows. 



In my recently published memoir on some ectoparasites of 

 Gavimarus pidex I have attempted to show that the known 

 freshwater Kotatoria fall under two natural subdivisions, which 

 differ from each other in the general form of the body and in 

 many peculiarities of organization. TJie most important con- 

 trast is manifested in the structure of the female generative 

 organs. In one group, that of the Aductifera, or Philodinse^e, 

 these are paired, possess no efferent ducts, and are composed 

 of a homogeneous plasma-mass permeated by nuclei*, definite 

 portions of wliich are gradually constricted off, and fall, as 

 ova, into the body- cavity. In the other group, the sexual 

 organs are unpaired, but are formed of two sections having 

 different functions, one of which (ovary) contains the cells 



* See the supplementary note at the close of this Memou". 



