294 WILLIAM A. HASWELL. 



very often gives the fibres the appearance of hollow tubes ; 

 when perfectly preserved it presents a reticulate appearance 

 such as is represented in fig. 3 ; but this is very rarely to be 

 observed. Within the sheath, besides the delicate substance 

 constituting the nerve-fibre, there are also in the commissural 

 nerves that form an important part of the cerebral ganglion, 

 though not, so far as I have observed, in the course of the 

 peripheral nerves, numbers of bipolar ganglion-cells. 



This arrangement of the nervous system is, as regards the 

 posterior part of it — the six longitudinal cords — very similar to 

 that described by Lang as observed by him in the Tristo- 

 midse. 1 The development of the tentacles in Temno- 

 cephala, and the consequent greater relative importance of 

 the anterior region of the animal, are accompanied by a greater 

 development of the nerves running forwards from the cerebral 

 ganglion. 



The single pair of eyes (PI. XXI, fig. 13) are of extremely 

 simple structure. They lie almost over the brain, so that the 

 nerves which pass up to them are very short. The eye consists 

 of a cup-shaped mass of dense pigment (p.), at the mouth of 

 which are one or two nerve-cells (g.) not differing from those 

 of the cerebral ganglion. Enclosed in the cup, the mouth of 

 which is directed upwards and outwards, is a highly refracting 

 body (r.) of a spherical form. This stains with difficulty and 

 not very darkly, and is obscurely fibrillar in minute structure ; 

 it contains a nucleus near the mouth of the cup, and towards 

 the base shows a trace of division into separate segments. 

 Completely enclosed in the substance of the pigment of the cup 

 on its inner side is a spherical cell (t.) } of nearly the same dimen- 

 sions as the body contained in the cavity of the cup ; this 

 exhibits a fine protoplasmic network and contains a solid- 

 looking nucleus. 



1 " Untersuchungen zur vergleichenden Anatotnie u. Histologic des Nerven- 

 systems der Platyelmiuthen," ' Mittheilungen aus der Zool. Station zu Neapel,' 

 ii Band. A very similar arrangement is described inDistomumisostomum 

 by E. Gaffron (" Zum Nervensystem der Trematoden," ' Zool. Beitrage,' 

 herausg. v. A. Schneider, 1884, known to me through an abstract in the 

 Biologisches Centralblatt,' iv). 



