CHAPTER XXV 



THE CEPHALOOORDA— AMPHIOXUS LANCEOLATUS 



In the first volume of this work the frog, a vertebrated animal, 

 was selected as an example of animal organisation, and in 

 studying it the beginner will have gained a tolerably clear idea 

 of the leading features of vertebrate structure. But the frog 

 is by no means a simple vertebrate : almost every detail of its 

 anatomy exhibits a complexity which forbids our regarding it 

 as a near representative of that primitive stock from which, on 

 the theory of evolution, we must suppose all vertebrate animals 

 to have sprung. Happily for the study of comparative anatomy, 

 we have an excellent example of a primitive vertebrate in the 

 remarkable fish-like animal Amphioxus, found somewhat 

 rarely on sandy bottoms in shallow waters off the English 

 coast, but more abundantly in other parts of Europe, especially 

 in the Mediterranean. Nor is Amphioxus confined to 

 European waters ; it has been found in shallow seas in almost 

 every part of the world, and several species have been 

 described, some exhibiting such well-marked characteristics 

 that they have been thought worthy of generic rank. The 

 common European species is named Amphioxus lanceolatus, 

 and it is the only one that will be referred to in the present 

 chapter. 



Although Amphioxus is fish-like- in form, it has neither the 

 structure nor the habits of a true fish. It has neither head 

 nor jaws, is destitute of paired fins, and has many anatomical 

 peculiarities that are not found in any fish. It can swim 

 rapidly in the water by sinuous movements of its narrow, 

 knife-shaped body, but in its most usual posture it is buried 

 vertically in the sand, the anterior end of its body projecting 

 into the water. In this position it obtains its nourishment 

 from the minute organisms drawn into its mouth in the 

 currents created by the ciliated apparatus to be described 

 further on. It has the most astonishing powers of burrowing, 



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