Developments of Parasites 137 



The Origin of Vertebrates 



The probable origin of the vertebrate type has been a 

 puzzle to biologists even if they readily accept the doctrine 

 of descent from different forms. In two generations, how- 

 ever, there have been accumulated facts regarding the de- 

 velopment of many types of animals; and these point to 

 affinities that had not been suspected. In the development 

 of the backboned individual, there is formed a gelatinous 

 rod running the length of the embryo, the notochord. This 

 serves as the basis for the subsequent development of the 

 backbone although it is not itself either a backbone or a 

 part of the backbone. There are some minute water animals 

 — " worms " in general appearance — which also develop a 

 supporting structure. These have been combined by some 

 zoologists with true vertebrates into a more comprehensive 

 group called Chordata. One of this group, the amphioxus 

 or lancelet (Fig. 37) shows very definite affinities to *' worm- 

 like " forms on the one hand, and to the early stages of 

 backboned animals on the other. Moreover, the sea squirts, 

 a group of sessile animals that we should be less likely to look 

 upon as possible ancestors of the vertebrates than clams, show 

 in their development very close resemblance to the amphi- 

 oxus. While nobody can say that the vertebrates as a group 

 are descended from the simpler chordates, or that the lancelet 

 and acorn worm are descended from an animal like the sea 

 squirt, these close resemblances during the early embryonic 

 stages are certainly impressive. We can only say that the 

 respective groups have a great deal in common early in life. 

 The Russian biologist, Alexander O. Kovalevsky, pointed out 

 that these groups have so much in common during the early 

 stages that the formation of degenerate adult sea squirts and 

 the development of young fish proceed normally from prac- 

 tically indistinguishable larval structures. 



Development of Parasites 



Many parasitic and degenerate animals present particu- 

 lar difficulties in our attempt to classify them. The barnacles 



