Evolution 



notochord and nerve cord and in general the same characters 

 that we described for Amphioxus. It is only later that the 

 creature settles down and assumes its final degenerate seden- 

 tary form. There can be no doubt that the tunicates have 

 been derived from some lancelet-like form, but the course of 

 their evolution has been unique. The type is the lost brother 

 of the vertebrate family, who has chosen a distinctly downward 

 path in life ; yet who has come to no miserable end, but lives 

 on, more or less successfully, in his lower social sphere. 



The round-mouths, including the lampreys and the hag-fish, 

 stand midway between the lancelet and the fishes, and therefore 

 constitute for us an important group. 



The lamprey is a fairly generally known eel-like creature, 

 of which there is a smaller fresh-water, and a larger salt-water 

 species, the latter reaching a length of about a yard. It is found 

 attached to, and feeding on, the dead bodies of fish, and less 

 frequently on living specimens. The hag is much more definitely 

 parasitic in its habits, and often occurs in the body cavity 

 of living fish. These forms were for long regarded as fishes, 

 and are sometimes even yet included in that group, but all 

 their characteristics point to a very much lower position in 

 the animal world than such a classification would indicate. 

 One of the most striking external differences is that the 

 round-mouths have nothing to represent the two pairs of 

 fins which occur uniformly in the fishes, and \vhich are, in a 

 true sense, the forebears of our own arms and legs. Even 

 more important than this is the absence of jaw r s. The mouth in 

 this group is a simple round opening, whose edges are armed with 

 pointed teeth, the latter, however, bearing no real resemblance 

 to the teeth of the higher animals. By means of these teeth, and 

 a pointed, tongue-like organ, and by suction, the round-mouths 

 are able to bore into the tissues of the animals on which they prey. 

 The absence of jaws and of extremities is, of course, a feature 

 which they share with the lancelet. Turning to the internal 

 structure, we may first observe that there is still no vertebral 



column, but only a simple, rod-like notochord, similar in its 



86 



