II. THE VERTEBRATES. 



FIG. 147. 



BRANCH VERTEBRATA (ver te bra'ta). 

 TUNICATES, FISHES, REPTILES, BIRDS, ETC. 



The forms which we are now about to meet, have the 

 body supported by an internal rod, which separates the 

 dorsally situate nervous system from the alimentary and 

 circulatory systems ; a structure quite the reverse of that 

 exhibited by the dorsally placed heart and ventral chain 

 of ganglia of the previous forms. Limbs, when present, 

 are never represented by more than two pairs, and gen- 

 erally segmentation is not exter- 

 nally apparent. Inasmuch as the 

 supporting rod is generally divided 

 into a series of segments, the so- 

 called vertebrae, these animals are 

 known as VERTEBRATES, in 

 contradistinction to the previous 

 forms, the INVERTEBRATES. 

 Here again, Nature presents us 

 with seemingly intermediate 

 forms, which prevent our draw- 

 ing definite lines of demarkation. 

 The so-called Balanoglossus 

 (bal a no glos'sus), long supposed 

 to be a worm, has lately been 

 shown to have, at an early period of its life, a short 



Bal a no gifts' su s minu' tus. A young 

 Balanoglossus, having three gill 

 openings, G. P, Proboscis, h, So- 

 called uotochord. C, collar, through 

 which the section is made, showing 

 relative position of notochord. 

 Nervous system, N ; and alimentary 

 tract, a. 



