CHAPTER LVI 



LIFE-HISTORIES, CARE OF EGGS AND YOUNG, AND 

 DWELLINGS, IN BACKBONED ANIMALS (Vertebrata) 



We have here to consider in succession the classes of Primi- 

 tive Vertebrates (Protochordata), Fishes (Pisces), Amphibians 

 (Amphibia), Reptiles (Reptilia), Birds (Aves), and Mammals 

 (Mammalia). 



LIFE-HISTORIES, &c., OF PRIMITIVE VERTEBRATES 



(Protochordata) 



Worm -LIKE Protochordates (Hemichorda). — The widely- 

 distributed species of Acorn -headed Worms [Balanog/ossits) are 

 marine forms which burrow in sand or mud. A viscid fluid exudes 

 ^ from the skin and glues 



the surrounding deposits 

 into a sort of indefinite 

 tube, which, however, is 

 not of a permanent nature. 

 Development is indirect, 

 and two sorts of larva are 

 known (fig. 946). The 

 simpler of the two is 

 somewhat egg - shaped, 

 and covered with short 

 cilia, there also being a 

 circlet of longer ones near the hinder end. This larva gradually 

 passes into the adult form without any striking metamorphosis. 

 It first of all increases in length, and becomes divided by trans- 

 verse furrows into three sections, which are destined to grow 

 into the proboscis, collar, and trunk, of the mature form (see 

 vol. i, p. 300). The gill-slits and other characteristic organs 

 soon begin to make their appearance. 



Fig 946. — LarvcE of Acorn-headed Worms {Balana^lossus). en- 

 larged. A, Simple form of larva. B, Tornaria: c, anterior ciliated 

 band: ?«., mouth; i.a., intestinal aperture; w.z'., water-vascular sac. 



