78 . S. GOTO. 



is moulded in accordance with the general asymmetry of the body; 



From the front of brain two pairs of nerves are given off f PI. 

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

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

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

 with the posterior nerves by a commissure, which arises at the common 

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

 with the external anterior nerve at a short distance from its origin. 

 The external pair mainly supplies the anterior suckers. 



The above description of the commissures and the anterior nerves 

 is based mainly upon observations on Tristomwn. In the other 

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

 cumstances be demonstrated in sections ; but I have not had enough 

 opportunity to examine fresh specimens, and therefore have not been 

 able to obtain a general view of the arrangement of the commissures 

 and their x'elations to the main nerves. 



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

 shall begin with the brain. 



In cross-sections of the body, the brain is seen to be a band- 

 shaped body arching over the alimentary canal (oesophagus, pharynx, 

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

 ly free from cells. The 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, but after all 

 only weakly, as in Microcotyle fusiformis (PL IV, fig. 6) arid Di- 

 clidophora sessilis (PI. XI, figs. 1 and 2). In most species of Microcotyle, 

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

 few nucleoli crowded around the brain (PI. IV, fig. 6), while no 



