of the Bay of NaijJes. 101 



ture, while in Paraset'son this is either entirely deficient or 

 very feebly developed ; lastly, the testicular tubes in the 

 species of Seison are placed ventrally to the stomach, in Para- 

 sei'son beside or over it. 



The copulation I have, unfortunately, been unable to 

 observe ; it would be interesting to find out something about 

 it, as from the mere knowledge of the male genitalia we can 

 scarcely form any idea of the process. That the ductus 

 ejacidatorius is everted through its narrow aperture of exit is 

 quite inconceivable, considering the entire absence of any mus- 

 culature which could effect this. It therefore hardly merits 

 this designation in Paraseiso7i, whilst in Seison it may bear it 

 with propriety. Several times, indeed, I had occasion to see 

 how the sperm-flasklets were driven outwards into the anterior 

 peduncular portion of the ductus by the cilia. From the 

 absence of any organ having the function of a penis, copula- 

 tion can only be effected by the juxtaposition of the sexual 

 apertures, the difterent position of which in the two sexes must 

 compel the copulating individuals to take up an unusual 

 position with regard to each other. It is very remarkable 

 that, notwithstanding the great number of animals that I have 

 seen, I have never found spermatophores or spermatozoa in 

 the female individuals. 



The water-vascular system of Paraseison presents nearly 

 the same constitution in both sexes, small differences only 

 being produced by its union with the efferent ducts of the 

 reproductive materials ; from the typical structure of the same 

 organ in the freshwater Rotatoria it differs by the possession 

 of a long lateral canal with a ca3cal termination. With the 

 exception of the tail it traverses all the divisions of the body 

 in the form of two tubes, which, however, are comparatively 

 easy to find only in the trunk, whilst in the neck and head 

 they are very liable to escape the notice of the observer, 

 owing to their small calibre and transparent texture. In 

 favourable specimens we see on each side, about the middle 

 of the head (fig. 6, z^), a faint undulatory movement, which 

 is caused . by a small cylindrical flicker-organ (" Zitter- 

 organ''^). This is seated upon a slender canal, which at this 

 particular point forms some loops, and which may also be 

 traced forward for some distance, although I could not succeed 

 in discovering its anterior termination in the head. The con- 

 tinuation of this tube backwards traverses the whole neck and 

 bears a second flicker-organ (fig. 6, z^) at the transition- 

 point of the head and neck. Throughout this space the 

 lumen of the canal continues of equal width. When the 

 vessel has passed into the trunk it approaches the ventral 



