THE LOWEST VEKTEBRATES 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 armoured 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, revolutionised our ideas concern- 

 ing 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 subking- 

 dom, 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 afore- 

 said worms. 



In the introduction to the Vertebrates given in the first volume we have indicated 

 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 position 

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

 lain by the bodies of the vertebrae. In our description of the fishes we have, how- 

 ever, seen that in some of the lower forms the vertebras are represented only by the 

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

 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 



