82 S. GOTO. 



would be in clanger of being swept away. But on this head nothing 

 definite can be asserted. On the other hand, the comparatively 

 perfect structure of the eye in Tr. molae may be due to the habit of 

 this species. For, according to Lang'^ and Monticelli^^ this 

 worm is met with very commonly on the surface of the body of 

 Ortlwgaiiscus mola, and is thus constantly exposed to light, which is 

 not the case in Tr. ovale. 



In cross-sections the brain as well as the nerves show the re- 

 ticulated appearance s6 universally described by students of the flat- 

 worms. In the brain of Tristomum, the meshes are completely 

 filled with nervous fibres, direct continuations of the processes 

 of ganglionic cells (PL XXIII, fig. -i ; PL XXIV, fig. 12 ; PL 

 XXV^, fig. 7 ; PL XXI, fig. 1); but in the nerves the fibres often do 

 not quite fill up the meshes, but leave a clear space between them- 

 selves and the walls of the meshes. In Tristomum, as has already been 

 mentioned, the fibres show less and less an affinity for stains as we 

 recede from the nucleus, and finally they can scarcely be distinguished 

 from the fluid that fills the meshes of the connective tissue ; so that 

 in this case the meshes of the nerves are seen in cross-section to be 

 merely filled with a very weakly staining, slightly granular substance 

 (PL XX, fig. 10). In Diclidophora the nervous fibres are very 

 slender, and in cross-sections are seen merely as minute dots (PL XI, 

 fig. 1) ; while in nearly all other species the nervous fibres are very 

 difficult to make out in cross-sections. The nerve itself, however, can 

 be followed with certainty for the greater part of its course, but becomes 

 indistinct in the posterior part of the body where, as already stated, the 

 mesenchyma consists almost exclusively of reticulated connective tissue. 



1). Lang— Z. c. p. 29. 



2). Monticelli— Intorno acl alcuni elininti del Museo Zoologioo delln E.. University, di 

 Palermo. Naturalista Sioiliano, An. XII, 1893. Estratto p. 5. 



