552 



THE CHANGING WORLD OF LIFE 



different from Amphioxus, and by studying only the adult type it might 

 be concluded that the two animals were not closely related. However, 

 as we learned in Chapter 25, the lamprey larva is so nearly like Am- 

 phioxus that the two were placed in the same subphylum until it became 

 known that the larval form changed into an adult that was already 

 placed in a different group. It would seem very unlikely that this simi- 

 larity is accidental ; it would seem more plausible that the two had com- 

 mon ancestors, but have diverged in their development. The similarity 



E F G H 



Animal Biology, Guyer, Longmans, Green & Co. 



Fig. 34.9. A series of embryos of different animals at comparable stages of em- 

 bryonic development. Note the close similarity between all of the early embryos, with 

 some distinctions appearing in the middle row, and individual species characteristics 

 becoming distinct in the later stages shown in the bottom row. A, fish; B, salamander; 

 C, turtle ; D, chicken ; E, pig ; F, calf ; G, rabbit ; H, man. 



of the worm-like larvae of insects to the annelids would indicate a similar 

 connection. 



The embryonic development of gill clefts among all the vertebrates, 

 as discussed in Chapter 21, seems to be significant. It is difficult to see 

 why the embryo of a snake should develop gill clefts, just as the embryo 

 of a fish, when the snake will never use gills, unless there is a relation- 

 ship between the two. The vertebrate heart, as studied in the same 

 chapter, has two chambers in the three aquatic classes, three in the 

 Amphibia, three with a partial partition in the ventricle in the reptiles. 



