Haswett—A Monograph of the Temnocephalee. 121 
The diagram of the arrangement of the dorsal nerve-cords and their branches 
given in my former paper (Pl. xxi. fig. 12) represents what is observable in 
T. Nove-zealandie. In young specimens of 7. fasczata recently hatched the course of 
these nerves is readily followed, and their arrangement, which is subject, as regards 
details, to considerable variation in different individuals, is represented in fig. 1 of 
Plate xm. In 7. guadricornis the arrangement is, in all essential respects, the same. 
In a horizontal series of sections single fibres from the transverse branches are seen 
to be given off with considerable regularity in a vertical direction, so as to perforate 
the muscular layers, probably ending in the integument. 
The second or dorst-lateral pair of nerve-trunks, which are much larger than 
the dorsal, are situated much nearer the lateral border, and are more deeply placed, 
though much nearer the dorsal surface than the ventral, and just outside the vitelline 
glands and the testes. The third pair or vez¢va/ cords are the largest of the three. 
They curve round the pharynx and run along the ventral aspect in the angle between 
the testes externally and the intestines internally. Connecting the ventral and 
dorsi-lateral cords of the same side, and connecting the two ventral cords are 
numerous transverse com missures. 
Only two of these three pairs of nerve-trunks, the dorsi-lateral and the ventral, 
were observed by Weber. He remarks on this point, ‘‘ Was ich von den nach hinten 
laufenden Nervenstiimmen sehe, ist mithin in vollstiindiger Harmonie mit Haswell’s 
fig. 6 auf Taf. xx., passt aber nicht zu seer Beschreibung.” The figure to which he 
refers is a general diagram of the organisation of Zemnocephadla, and in this, for the 
sake of clearness, the dorsi-lateral cords are omitted. 
The arrangement of the nerves, and in fact the entire structure of the nervous 
system of Zemmnocephala, conforms very closely to that of Z7zstomum mole as 
described by Lang*—the chief difference consisting in the greater development of 
the nerve-cords that pass forwards from the ganglion. This arrangement of the 
principal nerve-trunks is, however, not confined to the ectoparasitic forms, a similar 
disposition being observable in some at least of the Digenca.t 
The system of visceral nerves which extend through the wall of the pharynx 
was previously overlooked. It forms a system of some little complexity, the general 
arrangement of which is represented in fig. 3 of Plate xu. In the course of some of 
the branches are bipolar ganglion-cells. 
Eyes are absent in 7. /heringzz. When present they have in all the species 
which I have examined the general structure which I have described as characterising 
* “ Ueber das Nervensystem der Trematoden,” ‘ Mittheilungen a. d. Zool. Station zu Neapel,’ II. Band. 
+ E. Gaffron ‘*‘ Zum Nervensystem der Trematoden.” ‘ Zool. Anz.’ VI. (1883). 
R 
