CHORDATA. 



There are a few characters of the Vertebrates which are 

 shared by other forms. These features are: (1) the posses- 

 sion of gill-slits ; (2) a nervous system which is entirely on 

 one side of the alimentary canal; and (3) a notochord which 

 lies between the alimentary canal and the nervous system. 

 The existence of this notochord has given the name Chordata 

 to the assemblage. There are four divisions or branches of 

 the Chordata, only three of which need mention here. 



BRANCH I. LEPTOCAKDII (Lancelots) . 



The few species of lancelets (Ampliioxus) are all marine, 

 and occur in warmer seas. They have a body which is 

 fish-like, but they differ from all fishes in the absence of a 

 true heart and of a skull. The gill-slits are numerous 



FIG. 72. Diagram of Amphioxns (after Hertwig and Boveri). Above 

 (dotted) is the nervous system ; below it (cross-lined), the notochord ; the 

 mouth is surrounded by the circle of tentacles ; below the notochord is 

 the region of gill-slits ; the vent is near the posterior (right) end below. 



(about sixty), and these empty into a gill-chamber recalling 

 in some features that of the tadpoles. The notochord runs 

 the whole length of the body, and a stomach is lacking, 

 the liver emptying into the intestine just behind the gills. 

 Limbs or paired fins are absent, but there is a median fin 



153 



