476 VERTEBRATA. 



servation : the tentacula upon the head, the eyes, the respiratory 

 apparatus, and even the ink-bag, which in the earlier stages of 

 growth were quite undistinguishable in the germ of the future 

 being (fig- &18, 1), slowly make their appearance ; and, even before 

 birth, the little creature presents most of the peculiarities which 

 characterize the species to which it belongs. But the most pro- 

 minent feature that strikes the attention of the physiologist is 

 the remarkable position of the duct communicating between the 

 yolk of the egg, the great reservoir of nourishment provided by 

 nature for the support of the foetus whilst retained in the egg, and 

 the alimentary canal of the as yet imperfect Sepia. This commu- 

 nication, whicli in vertebrate animals is invariably effected through 

 an opening in the walls of the abdomen, whereby the vitelline duct 

 penetrates to the alimentary canal, here occupies a very unusual 

 situation ; being inserted into the head, through which it pene- 

 trates, by an aperture situated in the front of the mouth, to the 

 oesophagus, where it terminates (Jig- &18, 3). 



Leaving the Cephalopod Mollusca, we must bid adieu to the 

 fourth grand division of the animal kingdom, and proceed in the next 

 chapter to introduce the reader to beings organized according to a 

 different type, embracing the most highly-gifted and intelligent 

 occupants of the planet to which we belong. 



CHAPTER XXVI. 



VERTEBRATA. 



(517.) THE fifth division of the animal kingdom is composed of 

 four great classes of animals, closely allied to each other in the grand 

 features of their organization, and possessing in common a general 

 type of structure clearly recognizable in every member of the exten- 

 sive series, although of course modified in accordance with the 

 endless diversity of circumstances under which particular races 

 are destined to exist. The immeasurable realms of the ocean, 

 the rivers, lakes, and streams, the fens and marshy places of the 



