48 PHYLUM VERTEBRATA (CRANIATA). 



a ganglion, but, with the exception of the eighth, they differ 

 from them in containing a fair proportion of efferent nerve 

 fibres. The first and second cranial nerves, which supply the 

 special sense organs of smell and sight respectively, appear to 

 differ fundamentally from the other cranial nerves. 



In all Vertebrata there are three organs of special sense on 

 the head, the olfactory and visual, the innervation of which 

 has just been referred to, and the auditory, which is supplied 

 by the eighth cranial nerve. The connection of these organs 

 with the head has profoundly modified the structure of both 

 skull and brain. 



A special visceral nervous system, known as the sym- 

 pathetic, is nearly always present. 



The alimentary canal presents very similar features through- 

 out the series. It consists of a stomodaeum, pharynx, v oeso- 

 phagus, stomach, intestine. The stomodaeum contains the 

 teeth and the openings of the salivary glands if present, and 

 passes without any line of demarcation into the pharynx, which 

 in all Vertebrata is at some time of life connected with the ex- 

 terior by lateral apertures, the pharyngeal apertures or visceral 

 clefts. These are never more (usually less) than eight in number 

 on each side.* In Fishes and Amphibia the first of these (spiracle) 

 is always smaller than the others and may be completely absent ; 

 in the Amniota (Reptilia, Aves, and Mammalia) it is always 

 present, and not smaller than the others. In Fishes and 

 Amphibia the visceral clefts are used for respiratory purposes 

 as in other Chcrdata, but they are never put to this use in the 

 Amniota, where they appear to have no function at all. In 

 such cases the respiratory organ of the adult is the lung, which 

 is developed as a median outgrowth of the ventral wall of the 

 pharynx. There are two other nearly constant features 

 of the vertebrate alimentary canal, viz. (1) the connection of 

 two large glands, the liver and pancreas, with the anterior part 

 of the intestine, and (2) the connection of the generative and 

 renal organs with the hind end of the intestine, which is com- 

 monly called the cloaca. The junction of the endoderm and 

 ectoderm appears to take place at the anus, and there is prac- 

 tically no proctodaeum in the Vertebrata. 



* Except in some Marsipobranchs; 



