NEMERTEAN HELMINTHOZOA. 153 



the existence of an internal general or visceral cavity, which he invari- 

 ably found filled with a transparent fluid kept in continual agitation by 

 the various movements of the animal. The flux and reflux of this fluid 

 are rendered conspicuous by the movements hither and thither of nu^ 

 merous round diaphanous corpuscles which enter as morphotic elements 

 into its composition and render indisputable its identity of character 

 with the chylaqueous fluid of the more highly organized worms, in which 

 the internal viscera are freely suspended, their parietes being merely 

 kept in situ by delicate membranous frena. 



(406.) Besides the arborescent tubes in which digestion is accom- 

 plished, a rudimentary vascular system is distinctly visible, whereby 

 the nutritive juices are dispersed through the system. This consists of 

 a delicate network of vessels, arising from three large trunks, one placed 

 in the centre of the dorsal aspect, and the other two running along the 

 sides of the animal (fig. 75, 2). 



(407.) The Planariae are perfectly androgynous, as each individual 

 possesses a distinct male and female generative system ; but they are 

 not, apparently, self-impregnating, seeing that the cooperation of two 

 individuals has been found needful for the mutual fertilization of their 

 ova. In every one of these animals two distinct apertures exist upon 

 the ventral surface, at a little distance behind the root of the proboscis, 

 the anterior of which gives issue to the male organ, while the posterior 

 leads to the ovigerous or female parts. 



(408.) In Planaria tremellaris, the penis, which during copulation is 

 protruded from the anterior orifice (fig. 75, 6), is a white contractile 

 body, enclosed, when in a retracted state, in a small oval pouch ; it is 

 perforated with a minute canal, and receives near its root two flexuous 

 tubes, which gradually decrease in size as they diverge from each other, 

 until they can no longer be traced. These are the seminiferous vessels 

 (fig. 75, 5, a). The posterior genital orifice, which leads to the female 

 organs, communicates with a small pouch, or uterus, as it might be 

 termed (fig. 75, 5, &) ; into this open two lateral oviducts, which run on 

 each side of the male apparatus and of the proboscis ; these are very 

 transparent, and only recognizable under certain circumstances by the 

 ova which they contain. In Planaria lactea the oviduct opens into the 

 uterine cavity by a single tube, which, passing backwards, divides into 

 two equal branches ; and both of these, again subdividing, ramify ex- 

 tensively among the ceeca derived from the stomach. We likewise find 

 in this species two accessory vesicles, which pour their secretions into 

 the terminal sac. 



(409.) The NEMERTEAN HELMINTHOZOA are marine animals, frequently 

 found lurking beneath stones, or in clefts of rock, on shores left by 

 the retreating tide. In their external appearance they have some re- 

 semblance to Tseniae, and, like them, are occasionally met with of pro- 

 digious length. In their anatomical structure, however, they differ 



