30 BRAIN AND BODY OF FISH 



the facial lobe is large and separates the anterior ends of the vagals. 

 In fact the tench has some of the characteristics of Group III, as 

 will be evident when we recall the facts given in the last chapter, 

 when describing the division of the facial nerve. 



Group II. — Roach, Rudd, Dace, Chub. 



The optic lobes are large when compared with the vagal and 

 facial lobes. The vagal lobes are small, and the facial very small, 

 but is really larger in depth than would appear from a superficial 

 examination ; this is evident when serial sections are examined. 



Group III. — The Barbel, Gudgeon and Loach. 



In these three fish the vagal lobes are well-marked, but the facial 

 lobe is very much enlarged, particularly so in the gudgeon and the 

 loach ; but it will be found that the facial lobe of the barbel has 

 a very considerable depth, and it widely separates the anterior ends 

 of the vagals. In this group it will be noticed that the optic lobes 

 are separated posteriorly, especially in the barbel. This is due to an 

 extension of the cerebellum, inserting itself between the tecta optica, 

 which form the roof of the optic lobes. This extension of the cere- 

 bellum is known as the " valvula cerebelli." As valvula has a very 

 different application in human anatomy, its use here is unfortunate. 

 The " valvula cerebelli " is peculiar to fish and its function is specu- 

 lative. It is small in the cod, larger in the carp and still larger in 

 Group III, reaching its maximum in the barbel among the Cyprinoids. 

 It is of immense size in a family of African fishes, known as the 

 Mormyridse, which we shall have to describe in a later chapter, 

 but we may note here the great development of the snout. The 

 question of its significance may some day be solved by the study 

 of its comparative anatomy ; for the present the only clue seems to 

 be, that it is most developed in ground-feeding fish with a snout- 

 like proboscis. 



Mormyrus has a small mouth at the end of a more or less 

 elongated snout, so that it is sometimes called the elephant fish. 

 " The barbel has a rather long snout, with the uj^per profile decurved, 

 and the gudgeon is rather similar to the barbel in general form as 

 well as in the shape of the head " (Regan), and both grope and grub 

 for their food. 



Although the feeding habits of British Cyprinoids are similar, 

 in that they subsist on a mixed diet, some are almost entirely 

 vegetarian, while other are predacious. They can however be 

 divided into three groups according to their diet. 



