106 



SUB-CLASS MARSIPOBRANOHII (CYCLOSTOMATA). 



-IS 



inspiration by the dilatation of the branchial region caused 

 by the elasticity of the branchial basketwork. The double 

 valve (velar fold) at the junction of the mouth and branchial 

 portion of the alimentary canal prevents the regurgitation of 

 water in expiration. 



In Petromyzon it has been ob- 

 served that in every inspiratory 

 and expiratory movement of the 

 muscles of the branchial region 

 water is at the same time taken 

 in and expelled from the nasal 

 opening. 



The central nervous system 

 is constructed on the usual 

 vertebrate type. The brain of 

 Petromyzon (Fig. 55) is unique 

 amongst Vertebrata for the fact 

 that the median part of the roof 

 of the sylvian aqueduct (iter) 

 is epithelial and covered by a 

 choroid plexus. Moreover, the 

 cerebellum is very small, and 

 the thalamencephalon of some 

 length. The third ventricle di- 

 vides in front into a right and 

 left canal, each of which, passing 

 laterally, gives off a branch back- 

 wards into the cerebrum and 

 forwards into the olfactory lobe. 

 The pineal body, or epiphysis, 

 consists of two vesicles, lying, 

 the one dorsal to the other, over 

 the anterior part of the thala- 

 mencephalon. The larger dorsal 

 vesicle is the pineal body proper, the ventral smaller one being 

 called the parietal organ. The dorsal vesicle lies close beneath 

 the skull-wall, and is the so-called pineal eye. The ventral 

 part of its walls contains a white, sometimes a black, pigment, 

 and presents a structure which recalls that of the retina. It is 

 connected by a solid stalk (pineal stalk) containing nerve- 



Fio. 55. — Dorsal view of the brain of 

 Petromyzon fluviatilis (after Ahlborn). 

 1 olfactory nerves : 2 left ganglion 

 habenulae (the two pineal bodies 

 have been removed) ; 3 continuation 

 of 2 along the roof of the third ven- 

 tricle ; 4 swollen termination of 3 

 which is connected with the ventral 

 of the two pineal bodies ; 6 fourth 

 ventricle ; 6 edge of thin roof of fourth 

 ventricle ; 7 cerebellum ; 8 optic 

 lobes ; 9 edge of thin roof of iter ; 

 10 posterior commissure ; 11 right 

 ganglion habenulae ; 12 cerebral 

 hemisphere ; 13 olfactory lobe. 



