DISTOMUM CIRRIOERUM. 289 



end of tlie embryo. These later on, when the animal is 

 emerging from the cercaria-cyst, are arranged in two irre- 

 gular longitudinal lines, and in a transverse line a little 

 below the level of the ventral sucker. They become the 

 yolk-glands (Fig. XII, yl. gl.). 



Excretory System. — At the posterior end of the body 

 there is differentiated a V-shaped tract of cells with the apex 

 touching the cortex. The tract becomes split, and forms a 

 V-shaped vesicle (Figs. XII, XIII, ex. hi). 



The coi'tex also splits and the general cuticle is continued 

 inwards, so as to line the exit-aperture of the bladder. As 

 the animal grows the exit-apertitre lengthens and forms the 

 tortuous channel lined by thick cuticle which occurs in the 

 adult. The cells of the bladder at the time the embryo 

 emerges from the cyst are quite flat. Even at this early 

 period a few perfoi'ated cells can be found ; these are 

 destined to form the intra-cellular ducts, but flame-cells could 

 not be identified. 



The general body parenchyma consists of vesicular cells 

 and branched cells. 



The muscular cirrus-sac expands greatly, and the enclosed 

 sac of flat cells forms the vesicula seminalis, the exit-duct, 

 and the cirrus. 



Nervous System. — The nervous system commences its 

 development as three small collections of cells, one on each 

 side of the pharynx, and one dorsal to it. The cells mostly 

 produce fibrous elements ; the number of ganglion cells 

 formed is very few. The lateral masses become the cerebral 

 ganglia, and the dorsal mass forms the dorsal commissure 

 (Fig. XVIII, c.^.,cLc.). 



Laurer's canal can now be identified as a very thin duct 

 running out to the dorsal surface. The cortical layer of 

 protoplasm becomes relatively much thinner, and the cuticle 

 with spikelets regularly arranged becomes thicker. The cells 

 which produced the cortex become the so-called epidermal 

 cells. Between these cells long thread-like cells Avith flat- 



VOL. 47, PART 3. NEW SEKIES. T 



