The Lancdet. 5 



through which the water which enters the mouth 

 escapes into the space between the wall of the body 

 and that of the pharynx. To this space the name 

 ' atrium ' is given, and it opens externally by a median 

 pore or outlet placed on the under edge, and in front 

 of the end of the intestine (fig. i,/). 



The liver is a simple sac, and the heart is a single di- 

 lated tube(^), like that of some worms; it sends branches 

 backwards to the pharyngeal wall, one along each slit, 

 and these join dorsally below the notochord, making 

 a dorsal aorta or large blood-vessel, which gives off 

 branches to the different parts of the body. 



Though there is no brain, yet two of the foremost 

 of the many nerves emitted from the spinal marrow 

 supply structures which may be regarded as rudimen- 

 tary sense-organs. Thus there is in the middle of the 

 foremost end of the animal a small pit, possibly an 

 organ of smell, and two or more lateral pigment- spots 

 in front of and above the mouth, which may be organs 

 of sight. There is a narrow membranous fringe or fin 

 around the tail, but there are no limbs, and the blood 

 is colourless. 



The division of Vertebrata which contains this sin- 

 gularly aberrant form is named Acrania (headless), to 

 distinguish it from that which includes the entire re- 

 maining series, which is called Craniola (head-bearing), 



