CHAP. IV AMPHIOXUS HISTORY I I 3 



1875 and in 1889, Eetzius in 1890, and Boveri and Hatschek, 

 both in 1892. Important papers on special points have also 

 been written by Eolph, Eohde, Benliam, Andrews, Goodrich, and 

 others. The development was first elucidated by Kowalevslcy in 

 1867, at about the same time when he studied the development 

 of the Ascidians, and later again in 1877. Further papers on 

 the development and metamorphosis we owe to Hatschelc in 

 1881, Lankester and Willey in 1890 and 1891, Wilson in 1893, 

 and quite recently to MacBride. Dr. Willey 's book, Amplnoxus 

 and the Ancestry of the Verteirata (1894), contains a summary 

 of investigations on structure and development, an interesting 

 discussion of the relations of Amphioxus to the other Chordata, 

 and a full bibliography. 



In addition to such original researches, Amphioxus is 

 studied in more or less detail every year by countless senior and 

 junior students in zoological laboratories and marine stations 

 throughout the civilised world. The value of this primitive 

 form as an object of biological education depends upon the fact 

 that it shows the essential Vertebrate characters, and their 

 mode of formation, in a very simple and instructive condition. 

 Although no doubt somewhat modified, and possibly degenerate 

 in some details of structure, in its general morphology it 

 presents us with a persistent type probably not far removed 

 from the ancestral line of early Chordata. There are no sufficient 

 grounds for the view that Amphioxus is a very degenerate re- 

 presentative of fish-like A^'ertebrata. 



General Characters. — The Cephalochordata (or Acrania, in 

 contradistinction to the Craniata or Vertebrata) are marine, 

 non -colonial Chordata, in which the notochord extends the 

 entire length of the body, running forward into the snout beyond 

 the nervous system. There is no skull, and the notochord 'is 

 not surrounded by any vertebral column. There are no limbs 

 nor paired fins. There is no exoskeleton, and the ectoderm is 

 a single layer of non -ciliated columnar cells. The mouth 

 is ventral and anterior, the anus is ventral, posterior, and 

 asymmetrically placed on the left side. The pharynx is a large 

 branchial sac, having its sides perforated by many gill-slits, and 

 is surrounded by an ectodermal enclosure, the atrium, which 

 opens to the exterior by a median ventral atriopore. The 

 stomach gives off a simple saccular pouch, the liver, which has 

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