No. 470] 



UNITY OF GNATHOSTOME TYPE 



81 



of our 1-ecent knowledge, that amphioxus is a true vertebrate, lack- 

 ing it may be the first trace of the craniate skeleton, and lacking 

 many of the other features which are characteristic of most of 

 the existing vertebrates, but is nevertheless, the only existing form 

 which serves as a connecting link between the simple ancestral 

 type of structure and the more complex anatomical and physi- 

 ological conditions of the higher vertebrate. 



Some zoologists have recently re-uttered Semper's opinion, that 

 amphioxus is not a true vertebrate, but such restatement of Sem- 

 per's opinion is justly to be compared to the restatement of the 

 opinions of the older zoologists, who at various times held amphi- 

 oxus to be a worm, a mollusc, and a tunicate. Instead of making 

 assertions as to what amphioxus is or is not, the only scientific 

 method of solving the problem of its actual position in the animal 

 series is by a careful study of its structure and a comparison of 

 this with the structure of the lower vertebrates. Such comparison 

 proves beyond the shadow of a doubt the relation of the amphioxus 

 to the lowest fishes. 



A review of some of the salient features of amphioxus' anatomy 

 will not be out of place at this point. As regards the form of the 

 body, all zoologists recognize the fact that the shape of the verte- 

 brate body is a result of the direct response of the organism to its 

 environment, particularly the necessity of locomotion. The lancet 

 shape of amphioxus is due to its burrowing habit. The lack of 

 paired appendages is due to the fact that amphioxus represents 

 the stage of the development of vertebrate structure when such 

 appendages had not yet been developed. The median fin folds 

 are well developed, both in the head and in the cau(hil re«,non, and 

 serve the same function in essentially the same way that they do 

 in other vertebrates. There is no trace of a quasi lateral fin fold, 

 nor of the buds of lateral appendages in the amphioxus, but neither 

 of these are found in the Cyclostome fishes, which are much more 

 highly organized than amphioxus. 



The lack of the development of paire.l npi)en.lages in amphi- 

 oxus and the Cyclostomes is not a mark of degradation, or degen- 

 eration, as some zoologists would put it, because the whole course 

 of their development and the facts of their morpholog}' prove con- 

 clusively that these structures were not called forth by the response 



