Johnston, Morphology of the Head. 191 



by the fact that in urodeles (Necturus, 108 ; Amblystoma) 

 there are a large number of pouches in the wall of the gut in 

 the trunk, segmentally arranged and resembling the gill pouches 

 at the time of their formation. These are probably to be in- 

 terpreted as vestiges of gill slits which formerly existed through 

 a considerable portion of the trunk. Such seems to be the case 

 in Bdellostoma stoiiti, where Price (109) has described thirty- 

 five gill slits in the larva, the last 10-14 o^ which persist as the 

 adult gills. The last gill slit lies in the same segment with the 

 twenty-ninth spinal ganglion. The writer does not think that 

 the great number of gills in Bdellostoma all belonged originally 

 to the head and that their extension into the trunk is due to 

 the shifting forward of trunk myotomes into the occipital re- 

 gion. The simpler explanation is much more probable ; name- 

 ly, that there has been a reduction of the anterior gills on ac- 

 count of the parasitic life of the animal. The persistent gills 

 really belong to the trunk and are homologous with the gut 

 pouches seen in the trunk in urodeles. 



e. The number of gill slits in existing vertebrates varies. 

 At the so-called critical stage in Amphioxus there are 14, and 

 this is usually regarded as the primary number for this animal. 

 In Bdellostoma stouti 35 are formed, 10-14 persist ; in other 

 species of Bdellostoma there are C-j, 11-12, and 14 gill sacs; 

 in Petromyzon 8 larval, 7 adult ; in Heptanchus 7 ; in Hexan- 

 chus 6 ; in most selachians 5 ; in amphibia 4. It is not possi- 

 ble to draw from these figures any conclusion as to the typical 

 number of gill slits for vertebrates. The writer thinks it very 

 probable that the number of gills in primitive vertebrates was 

 large and variable, even within the same species. 



f. The lining of the pharynx is innervated by visceral 

 sensory (fasciculus communis) components found in the VII, 

 IX, and X cranial nerves. 



4. Segmentation of the central nef^vous system. 



Owing to numerous modifications of organs in the special- 

 ized head, the cranial nerves have become so changed from 

 their primitive condition that the study of their segmental ar- 



