78 S- GOTO. 



in moulded in accordance with the general asymmetry of the body. 



From the front of l)rain two pairs of nerves are given ofï (^Pl. 

 XX, hg. 1), the inner of which proceeds towards the corners of the 

 anterior Ijorder, its branches supplying mainly that part of the body 

 which lies between the anterior suckers. The outer pair is connected 

 with the posterior nerves by a commissure, wliich arises at tlie common 

 base of the two posterior nerves, and curving a httle outwards, unites 

 with the external anterior nerve at a short distance from its oriij^in. 

 The external pair mainly supplies the anterior suckers. 



The above descri[)tion of the commissures and the anterior nerves 

 is based mainly upon observations on Tristonnnii. In the other 

 genera, also, the connnissures are present and can under favourable cir- 

 cumstances be demonstrated in sections ; but I have not had en(3ugh 

 opportunity to examine fresh specimens, and therefore have not been 

 aljle to oljtain a general view of the arrangement of the commissures 

 and their relations to the main nerves. 



I sliall now proceed to the histology of the nervous system, and 

 sliall begin with the brain. 



In cross-secti(jns of the body, tlie 1)rain is seen to be a l)and- 

 sliaped body arching over the alimenttiry canal (oesophagus, pharynx, 

 or mouth-cavity, as the case may be), and in many species is entire- 

 ly free from cells. T'lie greater part of its substance consists of 

 connective tissue, and, being consequently slightly stained, is 

 very conspicuous in cross-sections. In some species, however, it 

 contains besides the reticulated connective tissue a finely granular 

 substance which stains more deeply than the former, 1)ut after all 

 only weakly, as in Micvocoiijh' fnslfonms (PI, \\\ fig. (>) and 1)1- 

 cliilnpliora sessiUfi (PI. XI, hgs. 1 and 2). In most species of Microcott/Ie, 

 there are numerous, well staininir, rounded nuclei containing one or a 

 few nucleoli crowded around the brain (PI. IV, fig. (>), while no 



