A PARASITE OE THE TIGER SHARK. 
551 
and a small vessel, the latter nearer the margin. The course of the 
larger one is very tortuous, so that in moderately thin transverse sec- 
tions it often appears as a double vessel (figs. 20, 21). The tortuous 
course of one of the large vessels is shown in fig. 23. The course of 
the smaller vessels is sinuous. Branches of the water-vascular system 
extend to the apex of the scolex (fig. 14). Lateral branches enter and 
ramify through the primary folds of the pseudoscolex (fig. 111'). Tow- 
ards the base of the scolex the neck is traversed by four vessels (fig. 
10). A little further back, at the beginning of the cervical outgrowths, 
they are poorly defined (fig. 11). The interior of the neck at this point 
is made up of spongy tissue composed of interlacing fibers, and lateral 
vessels may be seen entering the cervical outgrowth. Fig. 11, l', shows 
the first indication of one such lateral branch, which becomes quite 
evident a few sections farther on in the series. Towards the base of 
the pseudoscolex the lateral branches again unite with the central 
longitudinal vessels of the neck (figs. 17 and 22). At the point where 
the neck begins to broaden abruptly the aquiferous vessels are larger 
and appear to form a kind of plexus of vessels (fig. 19, V). The exact 
disposition of the aquiferous vessels in the neck, pseudoscolex, and 
scolex have not been made out satisfactorily. Fig. 25 is a diagrammatic 
representation of what appears to be their general disposition. Hear 
the anterior end of the body the smaller of the marginal vessels are 
from one-third to two thirds the diameter of the larger vessels. As the 
vessels proceed towards the posterior end the large vessels increase 
in size very much, while the small vessels are relatively much smaller 
than they are near the head. 
In sections made about the middle of a large specimen, a large aquif- 
erous vessel measured 0.135 rara by 0.162 mm in its two diameters ; the 
smaller vessel of the same pair measured only 0.022 mm by 0.030 mm in its 
two diameters. Hearer the posterior end a large vessel measured 
0.162 mm in diameter ; its small companion only 0.016 mm , or but' one-tenth 
as much. In segments in which the sexual organs are mature only the 
large vessels persist, and they, too, are merged in the general body 
cavity in ripe segments. The aquiferous vessels are surrounded by a 
proper wall which is quite thin, with a few nuclei surrounding it. 
Nervous system . — A cluster of nucleated cells, the largest of which 
measured 0.06 mm in diameter, in a finely granular mass and lying cen- 
trally in the scolex about on a level with the anterior loculi, has been 
interpreted as the anterior development of the nervous system (figs. S, 
14, and 15). This mass is traversed by exceedingly fine transverse 
and by coarser longitudinal fibers. Toward the base of the scolex the 
nerve tissue is collected into two marginal areas, in which there are a 
few nucleated cells on the peripheral side of the aquiferous vessels 
(fig. 10). In the neck their course can be traced as two cords or ves- 
sels of spongy, granular tissue, when seen in cross-sec tiou, or of finely 
striated spongy and granular tissue when seen in longitudinal section 
