106 



SUB-CLASS MARSIPOBRANCHII (CYCLOSTOMATA). 



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 regurgltation 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 Vertebrate, 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 f which, passing 



FIG. 55,-Dorsal view of the brain of 

 Petromyzon fluviatUis (after Ahlborn). 

 1 olfactory nerves : 2 left ganglion 

 habenulae (the two pineal bodies 

 have been removed) ; 3 continuation 



off n 



Wflrr j into 

 War< 



anrl 



of 2 along the roof of the third ven- forwards inrn flip nlfaofn- 



tricle; 4 swollen termination of 3 Jliaciory 



which is connected with the ventral 



of the two pineal bodies ; 5 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. 



pineal body, or 

 consists of two 



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- 



