GENERAL CHARACTERS 



405 



chord (Fig. 99, nch) 

 extends to the an- 

 terior end of the 

 animal, and there 

 are no paired hmbs. 

 A brain (l>r) can 

 hardly be said to be 

 present, and there 

 are no paired olfac- 

 tory, optic, or audi- 

 tory organs. The 

 pharynx is relatively 

 very large, and is 

 perforated by very 

 numerous oblique 

 gill - slits {br. d), 

 which do not open 

 on the exterior 

 directly, but are sur- 

 rounded by a cham- 

 ber, the atrium 

 (Figs. 99, atr, and 

 100 A, b) ; this 

 opens externally by 

 a pore (Figs. 98 

 and 99, atrp), and 

 though differing in 

 its mode of deve- 

 lopment from the 

 branchial chamber 

 of the tadpole (p. 

 207), has somewhat 

 similar relations. 

 There is no heart 



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