THE LOWEST VERTEBRATES AND THEIR 



ALLIES 



CHAPTER I 

 THE LAMPREY GROUP CLASS Cyclostomata 



TILL within recent years both the lampreys and the strange little creature 

 known as the lancelet were generally included among the class of fishes, which was 

 also taken to comprise a number of armored extinct forms, of which a brief notice 

 is given below. On the other hand, the marine animals commonly termed sea 

 squirts, but technically known as ascidians, together with certain aberrant worm- 

 like creatures, were classed with the great assemblage of so-called Invertebrates. 

 Anatomical and palaeontological investigations have, however, revolutionized our 

 ideas concerning the creatures in question, with the result that while the lampreys 

 are now separated from the fishes to form a class by themselves in the vertebrate 

 subkingdom, the lancelet and sea squirts, together with the above-mentioned 

 worm-like creatures are now regarded as forming a subkingdom by themselves, 

 known as the Semivertebrates, or Protochordata. The reason for the separation of 

 the lampreys from the fishes will be gathered when we come to that group; but we 

 must briefly notice in this place the considerations which have induced naturalists to 

 brigade in one group such very dissimilar creatures as the lancelet, sea squirts, and 

 the aforesaid worms. 



In the introduction to the Vertebrates given in the first volume we have indi- 

 cated the leading structural features of that group more especially as developed in 

 its higher members; among these one of the most important being the dorsal posi- 

 tion of the great nervous system, or spinal marrow, which in the higher forms is 

 underlain by the bodies of the vertebrae. In our description of the fishes we have, 

 however, seen that in some of the lower forms the vertebrae are represented only by 

 the original cartilaginous rod known as the notochord, from which they are devel- 

 oped by constriction in the higher types. To this we have to add that in the earlier 

 stages of their development all vertebrates possess gill slits, which persist in their 

 original condition only in the fishes and lampreys. Now the result of anatomical 

 investigations has been to show that the lancelet, sea squirts, and the aforesaid 

 worm-like creatures agree with the Vertebrates in the possession of a dorsally- 

 situated nervous system, of a notochord, and of gill slits; and thereby differ from all 

 other known animals. Consequently we may classify the animal kingdom as 

 follows: 



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