CHAP. IV AMPHIOXUS—HISTORY I13 
1875 and in 1889, Retzius in 1890, and Boveri and Hatschek, 
both in 1892. Important papers on special points have also 
been written by Rolph, Rohde, Benham, Andrews, Goodrich, and 
others. The development was first elucidated by Kowalevsky 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 Hatschek in 
1881, Lankester and Willey in 1890 and 1891, Wilson in 18938, 
and quite recently to MacBride. Dr. Willey’s book, Amphiowus 
and the Ancestry of the Vertebrata (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-hke Vertebrata. 
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-shts, 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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