30 
Proceedings of the Eoyal Society of Edinburgh. [Sess. 
The water- vascular system (PL III. fig. 1) consists of two longitudinal 
trunks, which can be readily seen both in the living animal and in 
sections. 
I know of no Turbellarian in which the main structure of the water- 
vascular system is so obvious. The two main ducts can be followed from 
the region of the eyes to the extreme posterior end, where they double 
forward upon themselves to open by means of two short transverse canals 
into the bladder. 
The wall of the bladder, as can be readily seen in transverse sections, 
contains parallel bands of muscle fibre (PI. III. fig. 3). 
Bresslau states that the bladder is “nichts anders als eine einfache 
Epidermis einstiilpung.” And he therefore considers that it offers a marked 
contrast to the end-bladder of the Plagiostomidse, which is formed by the 
union of the two ducts. It seems to me that the wall of the bladder is* too 
well marked off from the epidermis (1) by the absence of rhabdites, (2) by 
the presence of muscle bands, for this view to be accepted, and it appears to 
me to mark off Polycystis Goettei very distinctly from all other Kalypto- 
rhyncha. 
The Scottish form appears to agree in all essential points with the 
Polycystis Goettei of Bresslau. 
Whether, however, having regard : 
(1) to the extremely aberrant condition of its water- vascular system, 
(2) to the possible absence of a bursa seminalis, 
it would not be better to place Polycystis Goettei in a special genus, 
seems to me to be still a moot question. 
Microstoma lineare. 
This form was found in all the Scottish lochs which I have personally 
examined. 
The anatomy of the budding form has long been well known, and the 
only points to which I wish to refer in this note are — 
(1) The nematocysts. 
(2) The gonads. 
It has long been known that the nematocysts of Microstoma lineare 
are very like those of Hydra; and Yon Siebold, in his description of these 
organs, remarks that they are structures “welche denen der Hydra auf 
einen haar gleichen sollten.” 
Yon Graff believed that the nematocysts of Microstoma differed from 
