Ixxvi Introduction. 



as in Amj^hibia along" the outer edge, or as in many Teleostei, 

 along the anterior or inferior surface of the renal g-l mds. Uro- 

 genital canals are formed in Ganoidei of both sexes, and in the 

 males of ElasmobrancJiii. In tlie Teleostei, the ureters often 

 fuse into an azygos duct, which opens above, or behind, or to- 

 g'ether with the g-enerative duct^ but always posteriorly to the 

 anus. The urinary bladder may take the shaj)e of a bilateral 

 dilatation, as in the Sharks and Rays; or that of a vesica bi- 

 cornis, as in some Ganoidei ; or of that an azyg-os sac, as in many 

 Teleostei. 



The encephalon fills the brain-case in the embryonic Fish, but 

 subseqviently, by the disproportionate growth of the cranial walls, 

 it comes to occupy a very small space in the cavity which they 

 inclose, the intervening* space between it and the perichondrium or 

 periosteum, as the case may be, of the cranial vault being- filled with 

 a mass of loosely comj)acted tissue, richly laden with fat. The 

 membranes, in relation with the external and internal surfaces of 

 the cerebro-spinal centres, develope pigment cells as in Amphibia. 

 The nerve-centres are smaller in relation to the body in this than in 

 any other vertebrate class ; the relation of the encephalon to the 

 body is stated as being" on an averag-e as low as i to 3000 ; and 

 the spinal cord of a Sturgeon which weighed i2olb.s., has been 

 stated to have been no thicker than that of a Frog. The spinal 

 cord is ordinarily, but not always, devoid of any enlargements, and 

 of uniform diameter throughout its length, which is usually com- 

 mensurate with that of the spinal canal. The cerebellum is very 

 variable in size, but it sometimes, as in Sharks and in the Tunny, 

 attains a greater size relatively to the rest of the brain in this than 

 in any other vertebrate class. It is never bilaterally bilobed, as the 

 divisions of the brain placed anteriorly to it are. The optic lobes 

 are frequently in osseous Fish larger than any other division of the 

 brain, a proportion which they never attain to in any other class. 

 The diencephalon, the homologue of the optic thalami, fails in some 

 Teleostei^ as also in the Dipnoi, to be differentiated from the 

 mesencephalon or optic lobes behind, and the prosencephalon in 

 front of it; but in many other Teleostei, in the Ganoidei, and 

 ElasmobrancJiii, this division of the brain is considerably elongated 

 antero-posteriorly, and bounds a ' third ventricle ' by its two halves. 

 In the Sharks and Rays, the prosencephalon attains the preponder- 



