SARGENT: THE OPTIC REFLEX APPARATUS OF VERTEBRATES. 207 



In all these blind species Manthner's fibres are abnormally large. This 

 naturally comes about with the greater importance given the auditory 

 sense accompanying the degeneration of the visual sense. 



EsociDAE (94). In Tylosurus marinus the torus longitudinalis ex- 

 tends through the whole length of the elongated optic lobes. Several 

 branches of Reissner's fibre emerge from the ' Schaltstiick,' just an- 

 terior to the posterior commissure. Another fine branch has been 

 traced still farther forward nearly to the base of the ganglia habenulae. 

 In specimens 20 cm. long, Reissner's fibre has a diameter of 2.5 micra. 



Gasterosteidae (98). The mesencephaloa of Gasterosteus bispinosus 

 is short and very compact. The tectum is so thick and the torus so 

 largely developed that the mesocoele is greatly reduced. In the adult 

 the torus is crowded down and partly around the posterior commissure 

 (Plate 10, Fig. 69). The cells of the torus are small and closely 

 crowded. At the anterior end a bundle of ftbres passes out of each 

 torus lobe into the canal-like fissure between the torus and the posterior 

 commissure. The greater number^of axons from the torus converge into 

 close bundles in the anterior portion of the torus lobes (trt. tor. fhr. 

 Bets, a.) and, traversing the ' Schaltstiick,' enter the ventricle in the 

 median plane anterior to the posterior commissui'e. These different 

 trunks converge and fuse beneath the posterior commissure, and fai'ther 

 caudad are joined by the posterior trunk. There is some evidence that 

 a trunk made up of axons from the ganglia habenulae also joins Reiss- 

 ner's fibre (Fig. 69). The consolidated fibre (fhr. Eeis.) has in the ven- 

 tricles and anterior part of the canal a diameter of 1.2 micra (see also 

 Sargent, :03'', Fig. 22). 



Atherixidae (106). The optic reflex apparatus has been studied in 

 many individuals of the genus Menidia (species gracilis and notata). 

 Sections of a number of stages from 1 cm. to 10 cm. in length have 

 been cut in the three principal planes. In the youngest specimens ex- 

 amined the apparatus is already developed, and Reissner's fibre, though 

 very small, is sharply defined. 



The torus longitudinalis (Plate 8, Fig. 58) is of large size, its dorso- 

 ventral thickness being as great as that of the tectum, below which it 

 projects into the mesocoele, largely filling that space. As the torus 

 hangs suspended freely in the mesocoele, there is no lateral pressure on 

 it, as in the Salmonidae, and its flattening is consequently dorso-ventral. 

 The cells of the optic reflex apparatus make up the mass of the torus, 

 and are oval or nearly spherical in form, with their long axes lying in 

 the direction of the outgoing axon. In the young specimens the axons 



VOL. XLV. — NO. 3 6 



