186 MARINE AND FISHERIES 



5 GEORGE v., A. 1915 



definite outer boundary which is at least optically different from the rest of the 

 layer. 



Nervous System. — In plerocercoids of this size the nervous system is quite 

 well developed and essentially the same as that described for the adult by Benedict. 

 The nerve ring is found at a depth of about 120/f from the apex, but it is quite thin 

 especially where it passes between the large end-organ and the suckers which are 

 quite close together at this level (PL XX, Fig. 8) . At the points where the large 

 nerves supplying the suckers are given off the nerve ring is swollen to form ganglia. 

 From the ring two somewhat flattened cords course posteriorly to supply the body 

 of the plerocercoid. In the anterior part of the body they are situated in the cort- 

 ical parenchyma, while in the posterior region they approach the centre somewhat 

 and lie in the band of longitudinal body-muscles just outside the excretory vessels. 



Excretory System. — The excretory system at this stage is characterized by 

 the presence of a large number of flame cells and two longitudinal vessels, connect- 

 ing anteriorly with a meshwork of fine tubes surrounding the suckers and end- 

 organ. These two vessels are unequal in size, nor are their courses and connections 

 similar. The larger, averaging between 5 and 8fi in diameter, has thin walls and 

 gives off a large number of branches whose diameters are quite as great as that 

 of the main vessel. These branches are distinguishable as vessels with walls for 

 very short distances only, since they soon fuse with parenchymatous spaces in a 

 complicated manner. Here and there branches can be seen running from this 

 vessel to the periphery, narrowing as they approach the cuticle and eventually 

 piercing it by apertures much smaller than the diameter of the main vessel. These, 

 however, are not as numerous as might be expected from the development of the 

 main tube itself. The other vessel is from one-quarter to one-third the size of the 

 larger and pursues a straight course; on the other hand, it has thicker walls in 

 which prominent nuclei are to be seen. While its origin in the region of the suckers 

 is more easily made out than that of the larger vessel, posteriorly it becomes so 

 constricted at different levels that it all but disappears from view; near the caudal 

 vesicle, and just before joining the latter, it bends forward and inward behind the 

 anterior end of the vesicle and opens by an aperture quite separate from its fellow 

 of the opposite side. 



On each side of the plerocercoid the two excretory vessels are situated just 

 within the longitudinal body muscles, about 35/i apart and on a line inclined at 

 various angles to the perpendicular to the longitudinal axis of the transverse 

 section, the smaller vessel constantly lying nearer the centre of the section. 



The caudal vesicle is TOju long and 10^ in diameter, including the cuticular 

 lining. The lumen itself is somewhat stellate in shape owing to deep incisions and 

 folds in the cuticula. The absorptive cells follow the cuticula from the outer wall 

 of the worm throughout its whole length, while the cuticular muscles are well 

 developed as far as the apertures of the excretory vessels. As this place is 

 approached the longitudinal fibres diverge and become lost in the parenchyma; 

 likewise the absorptive layer suddenly disappears. 



Flame-cells are very numerous and comparatively large at this stage. They 

 are found to be confined to an area around the excretory vessels as described by 



