392 ^"^ ANIMAL KINGDOM 



annelids or arthropods, and the degree of resemblance of the proteins of 

 live animals has been shown to be a good measure of their evolutionary 

 relationship. The serological technique by which the degree of protein 

 similarity is determined is described in Chapter 35. 



Professor Berrill of McGill University has recently proposed that 

 primitive chordates were sessile, filter feeding, marine organisms not 

 unlike present-day ascidians. Gill slits presumably evolved in this group 

 as a means of concentrating food; a respiratory function for gill slits was 

 a secondary development. The tadpole-type larva, with its sensory ves- 

 icle and mobile tail supported by a notochord, evolved as a means of 

 selecting a suitable habitat for permanent settlement. Berrill postulates 

 that at a later time, and as an adaptation for exploiting the rich pas- 

 ture of oceanic surface waters, certain of these larvae became neotenic. 

 That is, they matured sexually but retained the other larval features and 

 failed to undergo metamorphosis. Contemporary, pelagic tunicates of the 

 class Larvacea have unquestionably evolved through neoteny, so this is a 

 reasonable proposal. Certain of these neotenic tadpoles came to exploit 

 the rich detritus at river mouths. An increase in size and in powers of 

 locomotion, particularly the evolution of a segmented muscular system, 

 would have enabled them to overcome the current and ascend the 

 rivers. The segmentation of certain chordates and of annelids and arth- 

 ropods is therefore attributed to the independent evolution of increased 

 activity in unrelated lines of descent. Berrill believes that vertebrates 

 gradually evolved in this way as a fresh-water adaptation of neotenic 

 tunicate larvae. He considers At?iphioxiis to be a relic of a phase in which 

 chordates were becoming more active and entering fresh water, but that 

 it has subsequently readapted to the life of a marine filter-feeder. 



It seems likely that vertebrates evolved from soft-bodied forms, and 

 the ancestral fossils may never be found. Thus direct paleontologic evi- 

 dence bearing on Berrill's and other theories of the origin of chordates 

 may never be available. 



Questions 



1. How does the nerve cord of chordates differ from that of nonchordates? 



2. What is the function of the notochord? 



3. What was the primitive function of the gill slits? 



4. Briefly characterize each of the chordate subphyla. 



5. Compare the method of feeding of Molgula and Amphioxus. 



6. List the eight classes of vertebrates and give an example of an animal that belongs to 

 each one. 



7. Make a diagram of a generalized vertebrate showing the arrangement of the major 

 organs and the features that distinguish it from other chordates. 



8. To which group of nonchordates are chordates most closely related? What is the 

 evidence? 



Supplementary Reading 



Excellent accounts of the lower chordates can be found in Parker and Haswell, 

 Text-Book of Zoology, and in Young, Life of Vertebrates. An older, yet very valuable 

 reference is Delage and Herouard, Traite de Zoologie Concrete, Vol. 8, Les Protocordes. 

 The urochordates are discussed thoroughly and interestingly by Berrill in his books. The 

 Tunicata and The Origin of Vertebrates. 



