144 NATURE STUDIES. 



culate the water admitted to the cavity. The throat 

 leads into a simple stomach (g), and this, in turn, 

 leads into the intestine (i). A liver (h) of the most 

 rudimentary description also exists. The blood, in 

 the absence of a heart, is circulated by the contrac- 

 tions of the blood-vessels, and it is interesting to note 

 that the arrangement seen in all vertebrates, whereby 

 a portion of the venous or impure blood is sent to 

 the liver for the manufacture of bile, is represented 

 even in this curious fish. When the lancelet breathes, 

 water is received into the throat, passes over the net- 

 work of blood-vessels in the walls thereof, and after 

 giving up its oxygen to the blood, is sent by the 

 ciliary action through the slits in the throat into the 

 body cavity. Thence it is expelled through a small 

 opening near the tail, and called the abdominal 

 pore (p). 



The lancelet differs from every other vertebrate 

 animal in the absence of a heart, skull, brain, and 

 kidneys, and in the colourless blood ; in the peculiar 

 and rudimentary liver; and in a digestive system, 

 which, like the throat, is also lined with cilia. But 

 that it is a vertebrate is proved by the presence of 

 the notochord (ri), already remarked as being deve- 

 loped in the early life of every vertebrate animal. 

 The supply of venous blood to the liver is also a ver- 

 tebrate character; whilst its development, studied 

 from the egg onwards, no less clearly shows its right 

 and title to be regarded as a true vertebrate. 



Passing now to a very different group of animals, 



