86 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



much as in a fish, we have gills developed at the 

 sides of the anterior region of the digestive tract. 



With regard to the Chordata, however, it is to be 

 distinctly borne in mind that l>otli these organ, 

 (notochord and gill-slits) are to be found, and we may, 



therefore, look 

 for the ancestral 

 or ideal Chordate 

 in an elongated, 

 bilaterally sym- 

 metrical, meta- 

 merically seg- 

 mented animal, 

 in which the cen- 

 tral nervous sys- 

 tem, dorsal in 

 position, was 

 supported by a 

 rod of firm tis- 

 sue, in which the 

 sides of the body 

 and pharynx 

 were perforated 

 by gill slits ; and 

 in which the 

 mouth was 



Fig. ^3. Young Balanoglossus seen from the side, placed Oil the 

 br. Branchial slits; x!2. (After Pagenstecher.) ventral Surface 



not far from the 



front end of the body. The Cliordata fall into 

 three well-marked groups ; in one degeneration has 

 proceeded to an extent so considerable, that in many 

 all indications of a chordate ancestor are completely 

 lost ; these are the Urocliordata or so-called 

 Tunicata. In another, many primitive characters, 

 such as the original segmentation and the notochord, 

 are retained unchanged, but in some few points 



