CHAPTER IX 



TYPE VII. CHORDATA 



The type Chordata (Lat. chordatus, having a cord) includes 

 all the remaining animals, and is necessarily a very large type, 

 although pretty sharply marked off from the others. For the 

 Chordata all possess three characteristics in common which are 

 found in no other type. First, the anterior portion of the ali- 

 mentary canal is in communication with the outside by a series 

 of slitlike openings in its walls; these are called gill slits, or 

 gill clefts, and occur at some stage in the life history of all of 

 the Chordata. Second, there is developed as an outgrowth 

 from the dorsal wall of the alimentary canal a rod of tissue 

 which is the foundation of the central or axial portion of the 

 skeletal system of the body ; this rod is called the chorda 

 dorsalis or the notochord ; it likewise appears at some stage 

 in the development of the animal and may disappear before 

 adult life. Third., the central nervous system lies exclusively on 

 the dorsal side of the alimentary canal. There are other charac- 

 teristics which apply to the type as a whole, but they can be 

 discussed more intelligently under the various subtypes into 

 which it is necessary to divide the Chordata. There are four 

 such subtypes. 



SUBTYPE I. ENTEROPNEUSTA 



The Enteropneusta (Gr. evrepov, intestine, and Trvevaros, 

 breathing) are represented by the genus Balanoglossus. Some- 

 what recently two deep-sea animals, Rhabdopleura (Fig. 269) 

 and Cephalodiscus (Figs. 270 and 271), have been associated 

 with it ; but as their position is still somewhat doubtful and 

 they do not appear to be of great systematic importance, they 



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