DISTOMIDiE. 21 



the caudal extremity, where they terminate in rounded coecal 

 ends. Immediately above, and in contact with the ventral sucker, 

 there is situated a small, circular papilla (e) which usually exhibits 

 two minute openings on its surface; these being, respectively, 

 the outlets of the male and female reproductive organs. One 

 of the openings in question communicates with a set of wide, 

 tortuous tubes (/) occupying the centre of the upper half of 

 the body. This uterine tube is usually of a deep yellow- 

 brown colour — a natural appearance due to the presence of 

 multitudes of small ova crowded together in the interior of 

 the convolutions. Below this greatly elongated uterus, there is 

 an irregularly oval mass situated directly in the mesial line ; this is 

 the ovary (g), and from it two narrow tubes may be seen passing 

 off, one towards either side of the body. The two ducts in 

 question are connected with the two botryoidal masses {i, i), or 

 yelk-forming glands, which are apparently situated between the 

 alimentary tubes and the lateral borders. Further down, below 

 the ovary, and in the central line, are the two testes (k, fc), the ducts 

 of which — as seen in an allied species here represented in a woodcut 

 (fig. 4) — pass upward in an undulating fine towards the reproduc- 

 tive papiUa. Lastly, there is the true excretory, or water-vascular 

 system, here represented by two straight (artificially-coloured) 

 red tubes (h, h) lying immediately in front of the yelk-forming 

 glands, and, at the lower part, curving downwards and inwards, so 

 as to coalesce in the central line, where they combine to form a 

 broad sigmoidal duct. In this situation the vessel hes between the 

 two testes, below which it suddenly increases in width, forming 

 what is called the contractile vesicle (I). Throughout its entire 

 course this system of vessels contains a fluid in which is sus- 

 pended a multitude of highly refi^acting granular particles ; these 

 latter, when they escape by the minute aperture placed at the base 

 of the contractile vesicle, constantly display the well-known 

 phenomena of molecular motion. A separate mass of these 

 glittering particles are shown (at fig. 5). In regard to the ova (figs. 



