160 ENTOZOA. 



somewliat roughly indicates the hmits of the superior third of the 

 body. lu some instances, one or other of these tubes is bifurcated 

 at some part of its course, but usually both of them pass in a 

 straight line to a point corresponding with the limiting border 

 of the testicular filaments, one to the right and the other to the 

 left. In this situation they divide abruptly into two secondary 

 channels, which also, on their part, each form a continuous line, 

 placed at right angles with the primary canal, and parallel with the 

 lateral margins of the body of the animal. Throughout their 

 entu^e course the secondary canals give off tertiary branches, which 

 pass outwards at a right angle to the former, and these again 

 subdivide into minute twigs, which ultimately terminate in bundles 

 of grape-like coecal extremities. These enlarged globular ends 

 constitute the so-called yelk- sacs, aiid all of them, combined 

 together, form the vitelligene glands which extend backwards 

 from the anterior part of the body — on a line with the ventral 

 sucker — to the caudal extremity. As shown in the drawing 

 (Plate XI., Fig. 1) they form a brownish-coloured horseshoe- shaped 

 mass, completely embracing the reproductive organs, and filling up 

 the central space of the lower third of the body, where the latter 

 is not occupied by the testes. In this situation the two vitel- 

 ligene masses do not coalesce, but are separated from each other, 

 in the middle line, by a very narrow space set apart for the 

 passage of the main canal of the water-vascular system. The 

 yelk- sacs themselves form little dots barely visible to the naked 

 eye, from. 55-5" to 250", and contain in their interior numerous yelk- 

 globules, or nucleated cellules, measuring about the -^^ of an inch 

 in diameter, the nucleus being wa". 



A nervous system has been described by Mehlis, Blanchard, 

 and especially by Leuckart. It consists, firstly, of a pair of 

 cephahc ganglia, situated in the space between the oral sucker 

 and the pharynx, but separated from the latter by a thin layer 

 of connective tissue. The ganglia give off two main branches 

 or lateral nerves, which run downwards in a parallel manner, 



