154 BRAIN AND BODY OF FISH 



similarity in size of their primitive end-brains, thus in Group II, 

 the end-brain of the lemon-dab, plaice, witch, and dab are larger 

 than those of Group III, the brill, turbot, and megrim. If we 

 compare the drawings of the lemon-dab and brill it will be seen 

 that both olfactory bulbs and primitive end-brain are more de- 

 veloped in the former than the latter. As Group II are bottom 

 feeders and feed largely by smell as well as taste, both elements of 

 the anterior lobes are functionally well develoj)ed. 



An examination of the plate giving the dorsal view of the three 

 types of Cyprinoid brain will reveal the same information as to 

 feeding habits as we have just noted in plaice and lemon-dab. 

 The barbel, gudgeon, loach, and tench have an elongated primitive 

 end-brain of considerable size, and these fish all feed largely by 

 taste and no doubt also by smell. The sight-feeding group as 

 typified by the roach have small end-brains, while the carp group 

 have end-brains intermediate between the above groups. 



Passing backwards we reach the mid-brain which superficially 

 appears as the optic lobes. These lobes are not so simply con- 

 stituted as would appear ; if the transverse section of the optic 

 lobes of the plaice be referred to, it will be seen that there is a 

 broad cellular layer of concentric bands of nerve tissue and fibres, 

 which is known as the tectum opticum, and that this forms a roof 

 to a cavity into which protrudes the base of the lobi inferiores and 

 the nerves passing dorsaUy from the nuclei rotundi. The optic 

 nerves passing from the internal surface of the tectum also 

 occupy the anterior part of the cavity on its lateral aspects. But 

 posteriorly as can be seen in the drawing of the brain of the cod 

 there is another protrusion or rather insertion of another structiu-e, 

 the valvula cerebelli, which not infrequently causes a wide separation 

 of the anterior ends of the tecta and exceptionally pushes its 

 way right through and envelops the optic lobes as in Mormyrus. 



We would recall in illustration of tliis condition, the optic lobes 

 of the various groups of carps ; it has been shown that those that 

 feed by sight have big optic lobes as the roach, but the barbel a 

 ground feeder has also a large optic lobe, but the wide separation of 

 the tecta optica by the valvula is the cause, as is very obvious in the 

 drawing, which illustrates these types of cyprinoid brain. The 

 loach is another good example of an apparently large optic lobe. 

 Nevertheless the fact remains true that fish that hunt in the dark, 

 as the sole, have small optic lobes, wliile those that hunt by day, 

 as the plaice, have larger lobes. But large eyes do not necessarily 

 mean that a fish will have well developed optic lobes. This fact 

 is observed in certain fish that frequent great depths, as the Black 



