406 THE TORUS LONGITUDINALIS OF THE TELEOST BRAIN: 



transversely occupies the greater part of its thickness (Fig. 8, dec. d.). It contains 

 few or no nerve cells, but consists almost wholly of ependyma. 



In the Gasterosteidse the mesencephalon is short and very compact. The tectum 

 is so thick and the torus and valvula cerebelli so largely developed that the mesoccele 

 is greatly reduced. The valvula is pushed forward into the mesoccele so that the 

 torus is crowded down and partly around the posterior commissure (Fig. 22). As 

 a result the torus is short and much thickened, extending through only half the length 

 of the mesencephalon. 



In the Atherinidse (Menidia) the torus longitudinalis is of large size, its dorso- 

 ventral thickness being equal to that of the tectum, below which it projects into the 

 mesocoele. As the torus hangs suspended freely in the mesoccele, there is no lateral 

 pressure on it as in the Salmonidse, and its form is consequently flatter and the lobes 

 remain distinct (Fig. 10). 



In the Scisenidse (Cynoscion) the optic lobes are of great size, and project forward 

 overlying the diencephalon. The torus longitudinalis is of large size and extends 

 through the length of the mesencephalon. It is suspended freely in the spacious 

 mesoccele entirely below the level of the tectum. The two lobes of the torus are 

 wholly distinct. Above the posterior commissure they have a peculiar and charac- 

 teristic form in transverse section, which may perhaps be likened to that of a comma 

 (Fig. 18). Posteriorly above the valvula they diminish in size and are kidney- 

 shaped in transverse section (Fig. 19). Anteriorly the lobes of the torus extend 

 forward into the lateral anterior horns of the mesoccele (Figs. 15, 16, 17). 



In the Labridae (Tautogolabrus) the lobes of the torus are relatively small; they 

 remain distinct and are separated by a wide median fissure, being connected only by 

 the thin bridge of tissue through which passes the dorsal decussation (Fig. 6). 

 Through the anterior half of the mesencephalon they are suspended below the level 

 of the tectum and have in transverse section a rounded cushion-like form. Poste- 

 riorly they taper away and come to lie more completely on the level with and between 

 the halves of the tectal roof (Fig. 7). Above the posterior commissure they fuse with 

 it, taking on a triangular outline in section. 



In the Pleuronectidse the torus is small and well developed only through the 

 anterior half of the mesencephalon. The torus lobes are distinct, flattened, and pad- 

 like, having a somewhat rounded rectangular outline in transverse section, and hang 

 in the mesoccele wholly below the level of the mesencephalic roof (Figs. 11, 12). 

 Posteriorly they are in close contact with the large valvula cerebelli (Fig. 13) and 

 taper away at about the middle of the mesencephalon (Fig. 14). 



