666 - Heredity and Evolution 



MOUTH 



ATRIOPORE 



ATRIUM 



ANUS 



GONAD 

 DUCT 



GONADS 



STOMACH 



ADULT 

 DIAGRAMMATIC SECTION 



LARVA ADULT 



EXTERNAL STRUCTURE 



Fig. 32-37. One of the urochordates (Tunicata). In the larval stage, but not in the adult, this 

 animal appears to be a typical chordate (see text). 



root, (3) the origin of the main digestive 

 gland as an outgrowth from the enteron, and 

 (4) the position of the major blood vessels. 

 All in all, therefore, it seems likely that the 

 surviving Cephalochordata originated from 

 the same ancestral stock as did the Vertebrata 

 and that they give the best hints as to what 

 the ancestral vertebrate stock was like. 



Subphylum 4: The Vertebrata. Vertebrates 

 (Fig. 32-39) were late starters in evolution. 

 In fact, none of the familiar modern kinds 

 left fossil remnants that can be dated earlier 

 than the Devonian period. Since then, how- 

 ever, their biological success has been im- 

 pressive. In the aquatic environment, which 

 many vertebrates have not abandoned, the 

 cartilaginous and bony fishes gave rise to 



many widely distributed dominant species. 

 But on land the success has been even more 

 spectacular. Many adult Amphibia are well- 

 adapted terrestrial species, even though they 

 almost always return to the water to lay their 

 eggs and to spend their embryonic and larval 

 (tadpole) stages. 



Vertebrate Characteristics. First and fore- 

 most, of course, vertebrates are chordate ani- 

 mals, and all display the classic chordate fea- 

 tures. The notochord, however, which can 

 easily be identified in the embryo, is always 

 replaced either partially or completely by a 

 segmented axial skeleton, the vertebral col- 

 umn. The notochordal material becomes 

 invaded by cartilage- or bone-forming cells, 

 which deposit the new skeletal material in 



Fig. 32-38. Amphioxus. This primitive cephalochordate animal provides some clues to the problem of how 

 vertebrates were evolved. (By permission from Integrated Principles of Zoology, by C. P. Hickman. C. V. Mosby 

 Co.) 



