NERVOUS SYSTEM. 213 



sion for the third ventricle. The brain exhibits a remarkable bend 

 from the mesocerebrum to the right. 



Another form of brain occurs moreover in the Cyclostomi. In 

 Petrornyzon we find a double (more or less fused) pair of ganglia, 

 from the anterior of which the nerves of smell arise ; they represent 

 the olfactory lobes and tubercles of the Osseous Fishes. Between 

 these and the pair of optic or quadrigeminal lobes, which are of 

 small size, is found the anterior part of the mesocerebrum, where a 

 single median lobe, that of the third ventricle (the pineal gland ?), 

 is developed. The cerebellum is in the whole genus uncommonly 

 small, consisting only of a transverse commissure connecting the 

 corpora restiformia. The inferior lobes are absent or rudimentary, 

 and developed near to the azygos lobes of the third ventricle. The 

 posterior lobes appear to be absent, though the medulla oblongata is 

 very broad superiorly and somewhat corrugated. 



The structure is still more abnormal and singular in Myxine and 

 Bdellostoma, where the brain is formed of several double divisions, 

 amounting to four pairs of ganglia, from the anterior of which arise 

 the olfactory nerves. A small median lobe belonging to the third 

 ventricle is also present. The cerebellum is absent, if the last bifold 

 division of the brain be not regarded as constituting it. The ventricles 

 are also absent. 



The brain of the Lepidosiren has been described in speaking of 

 the Amphibious Fishes (see p. 150). It is considerably developed, 

 being very similar to that in the naked Amphibia, especially the 

 Proteidea ; it has two lobes of the hemispheres, single quadrigeminal 

 masses, a lobe for the third ventricle (glanduia pinealis . ? ), a narrow 

 medullary band representing the cerebellum, and a bilobed pituitary 

 appendage. 



In the Vermiform Fishes, i. e. in Amphioxus s. Branchiostoma, 

 the rudiments of the higher senses are indeed present, as we shall 

 see further on, but the parts of the brain appropriated for the organs 

 of smell and sight are chiefly absent. The brain is not at all to be 

 distinguished from the spinal marrow ; the latter terminates obtusely, 

 and this part is to be viewed as the medulla oblongata, and as the only 

 appreciable part of the brain which is formed in this the lowest of ail 

 the Vertebrata. 



The Spinal marrow of Fishes does not exhibit nearly so many va- 

 rieties as the brain. It is generally almost cylindrical ; in the genus 

 Petromyzori, however, and some other Fishes, e. g. in its posterior 

 part in Chimera, it is flattened out like a band and extensible ; it 



