SEA-FISHEttlES LABORATORY. 249 



one would expect, seeing that it is connected with the 

 general activity of the organism, and the Plaice is sluggish 

 in habits. 



The mid-brain is represented on the dorsal surface on 

 each side by the tectum opticum (optic lobe). These are 

 very large bodies almost spherical in shape, and charac- 

 terised in dead and preserved specimens, and doubtless in 

 life also, by a deep furrow, which extends backwards in a 

 curve from the anterior margin of each lobe for about half 

 its antero-posterior diameter. 



As is usual in Teleosts, the 'iween-brain hardly 

 appears at all on the dorsal surface of the brain, being 

 excluded from it by the meeting of the two striata and 

 optic lobes. However, a small portion of its membranous 

 roof is visible, and from this there is seen emerging by 

 the triangular space formed immediately in front of the 

 median apposition of the two optic lobes, the extremely 

 fine pineal tube. In sections it is seen to arise as an 

 evagination of the roof of the third ventricle almost 

 behind the ganglia habenuke and in front of the posterior 

 commissure. It then passes forwards over the pallium of 

 the left striatum and swells into the large pineal gland 

 lying on the pallium near the anterior extremity of the 

 left striatum. By pressing apart the optic lobes there 

 may be seen immediately in front of the exit of the pineal 

 tube the ganglia habenulye and the plaited choroid roof of 

 the third ventricle. 



In a well-preserved brain the membranous pallium of 

 the fore-brain is very obvious. It is a large oval sheet, 

 with its long axis at right-angles to that of the brain, and 

 almost equal to that of the optic lobes. It is a very thin 

 membrane, and appears thicker than it really is on account 

 of the coagulated cerebro-spinal fluid in the ventricle. 

 The corpora striata are also visible through the pallium. 



