CHORDATES IN GENERAL 



149 



with water which is drawn in and expelled through a proboscis pore 

 or vent located on its dorsal side and just anterior to the collar. Sup- 

 porting the base of the proboscis is a short skeletal process which is 

 stiff and extends anteriorly from the roof of the mouth region and 

 assists in burrowing. This process, called the diverticulum, is usually 

 referred to as the rudimentary notochord. However, it is very poorly 

 developed and in a peculiar position. Nevertheless, it has the rela- 

 tionship to the digestive tube which is characteristic in the embry- 

 onic development of the notochord for certain higher chordates. 



Fig. 46. — External features of Dolichoglossus kowalevskii. 



Geppert Company.) 



(Courtesy of Denoyer- 



Apical plate 



Mouth 



[V Proboscis 

 ix :'/ coelum 



Anus 



Fig. 47. — Tornaria larva of Hemichorda. (From Hegner, Collerje Zoology, The 



Macinillan Company, after Metchnikoff. ) 



The mouth opens on the ventral side just anterior to the collar and 

 leads into the straight alimentary canal which extends to the pos- 

 terior end of the body and ends in the anus. Like the earthworm, 

 this animal utilizes the mud in which it lives for food, absorbing 

 the organic matter from it as nutriment. Balanoglossus has numer- 

 ous paired gill slits, located in the lateral walls of the anterior (sup- 

 posedly pharyngeal) position of the digestive tube. In some of the 

 other representatives the gills are much reduced in numbers or are 

 lacking. Where gills are present, water is passed through them for 



