THE LOWEST VEETEBEATES AND 
THEIE ALLIES. 
CHAPTER I. 
The Lamprey Group,— Class Cyclostomata. 
Till within recent years both the lampreys ancl 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 vertebrae 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 
