VERTEBRATA 



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The branch Vertebrata* is divided into the following 

 superclasses : 



No skull nor skeleton; notochord short, 

 anterior ; nervous center a longitudi- 

 nal cord; Hemichorda. 



No skull nor skeleton; notochord caudal 



only ; nervous center a ganglion ; Urochorda. 



No skull; notochord extending through- 

 out the body, included in a membran- 

 ous sheath, as is the cord-like nervous 

 axis above it; Cephalochorda. 



A cartilaginous or bony skull and skeleton, 

 which extends throughout the body ; 

 central nervous system a longitudinal 

 cord terminating in a brain within the 

 skull; Craniata. 



Superclass I.— HEMICHORDA. 



Helminthophya. 



There is but one class of Hemichorda: 

 Not metameric ; no atrium ; 



Class I.— HELMINTHOPHYA. 



Two orders of this class are known which differ as fol- 

 lows: 



*Tlie Vertebrata (or Chordata) as now extenfled, may be defined as coelomate 

 Metazoa, possessing a longitudinal, axial, cellular supporting rod (tlie notochordj. 

 derived from the hypoblast (except Hemichorda ?) and in the mesoblastic sheaths 

 of which the vertebral centra form in the higher types; with the walls of the 

 pharynx perforated by openings (gill slits, visceral clefts, stigmatse), placing its 

 lumen in communication with the exterior ; with an epiblastic central nervous 

 system which is in whole or in part tubular, which lies dorsal to the notochord and 

 which is not perforated by the functional mouth ; and with the mesoblast segmented 

 (obscurely in Urochorda). In the different groups any of these characters may be 

 persistent, or temporary and embryonic, or adaptively modified and obscure.— Ed. 



