PART II. THE ACRANIATA. 



CHAPTER XXII. 

 THE CRANIATES AND THE ACRANIATES. 



The Statement of the Problem. — One of the chief difficulties in the study 

 of the origin of vertebrates is to correctly estimate the value of contradictory 

 evidence and to assign due weight to opinions based on a particular point of 

 view, or on a particular source of information. Thus Amphioxus, Balanoglossus, 

 Cephalodiscus, the tunicates, echinoderms, and annelids have been variously 

 exploited as the ancestral stock from which the vertebrates arose, and each view 

 has had its followers that from time to time have openly confessed their faith and 

 duly readjusted their estimates of morphological values. 



From the very outset the analysis of the vertebrate head and trunk by means 

 of comparative anatomy and embryology seemed to demonstrate that the ances- 

 tral vertebrate was an elongated animal composed of many like metameres, each 

 one consisting of sharply defined members of the more important system of organs, 

 i.e., nephridia, neuromeres, myotomes, sense organs, gut pouches, and gill clefts, 

 thus suggesting the condition so clearly presented by many annelids. For many 

 years, therefore, Amphioxus was regarded as the most primitive existing verte- 

 brate, because its simple structure and the sharply defined segmentation of its 

 mesoderm, gill clefts, and other organs, appeared to represent the actual em- 

 bodiment of the ideal vertebrate. But the organs that are the first to show a 

 metameric arrangement in the invertebrates, and the ones to maintain it most 

 persistently, such as the appendages, sense organs, and nerve cords, were in 

 Amphioxus either absent or without any indication of segmentation. 



The fact that in typically segmented invertebrates, such as the arthropods 

 and annelids, the nerve cord nearly always consists of sharply defined and widely 

 separated ganglia, or neuromeres, while in Amphioxus little or no indication 

 of such a condition is visible, occasioned little comment, while much was 

 made of the segmental arrangement of the myotomes, gill clefts, and later of the 

 nephridia. 



When it was shown that the development of the tunicates was very similar 

 to that of Amphioxus, and that the tunicate larva had a well defined notochord, 

 which later disappeared during a process of degenerative metamorphosis, the 

 problem was greatly complicated, for the tunicates clearly belonged to a lower 

 type structurally than Amphioxus, yet they were farther removed from the hypo- 

 thetical, ideally segmented ancestor of the vertebrates than either Amphioxus or 

 any of the true fishes. 



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