ASCIDIANS AND LANCELETS 201 



apertures. The notochord extends the entire length of the body, 

 running forward into the snout beyond the spinal cord. There 

 is practically no brain, though two pairs of nerves which pass to 

 the sensitive front and above the mouth are different from the 

 other nerves and may be called cerebral nerves. What is some- 

 times called the " eye-spot " in front of the end of the spinal cord is 

 only a pigment spot, perceptions of light being produced by what 

 are termed " spinal eyes " at regular intervals right down the 

 spinal cord, and consisting of two cells, one a pigment cell, the other 

 a percipient cell. Water is taken in at the mouth, and expelled 

 through an opening in front of the anus called the "atrial pore." 



It will be seen, from the above brief description, that the 

 Ascidians, the Balanoglossus, and the Lancelet have certain 

 anatomical characteristics in common, which are also character- 

 istic features of all vertebrate animals, and therefore, although 

 destitute of any bony skeleton, these animals stand at the base of 

 the great phylum of backboned animals or Vertebrata. The 

 Ascidians, or Tunicates, are grouped by biologists under the head 

 of Urochordata, in which the notochord is restricted to the tail ; 

 Balanoglossus and its allies are grouped under the Hemichordata, 

 in which the notochord occurs in the anterior end of the body 

 only ; and Amphioxus comes under the Cephalochordata, in which 

 the notochord extends the entire length of the body and of the 

 head. The Vertebrata proper come under the head of the Craniata, 

 in which a brain is developed as an enlargement of the central 

 nervous system, while the notochord does not extend farther 

 forward than the middle of the brain, and a vertebral column is 

 present. The Craniata are divided into six groups, or " classes " : 

 I. The Cydostomata, Lampreys and Hagfishes ; II Pisces, true 

 Fishes ; III., Amphibia, Newts, Frogs and Toads ; IV., Reptitta, 

 Lizards, Snakes, Turtles and Crocodiles ; V., Aves, Birds ; VL, 

 Mammalia, Hairy Quadrupeds, which suckle their young. 



