296 CHARACTERS OF VERTEBRATE ANIMALS 



to the outside, but into a large atrial cavity which surrounds the 

 pharynx, and which itself opens to the exterior by a rounded 

 hole, the atriopore, on the under side of the body just in front of 

 the anal fin. The state of things may be more clearly under- 

 stood by reference to the development of the tadpole (p. 254), 

 where the gill-slits at first open directly to the exterior but are 



Fig. 171. Two Simple Ascidians. i, Ciona, intestinalis. 2, Phallusi'a mammillata 



later on covered over by the backward growth of a fold of the 

 body-wall, the upshot of this being the formation of a branchial 

 chamber into which the gill -slits open, and which opens to 

 the exterior itself by a small hole on the left-hand side of the 

 body. If this hole were in the middle line below, instead of on 

 the left side, it might be compared to the Lancelet's atriopore, 

 while the branchial cavity has much the same relations as the 

 atrial cavity. It would probably be incorrect to consider the 

 two cavities as closely equivalent, for they develop in very 

 different manners. 



The Lancelet will also stand the third test of a Vertebrate, 

 for it possesses a tubular nerve-cord, situated above the notochord, 



