MORPHOLOGICAL IMPORTANCE. 45- 



many other points, Amphioxus remains throughout life in a 

 condition characteristic of the early developmental phases of the 

 higher Vertebrates. Amphioxus halts permanently at a stage- 

 through which all the higher Vertebrates pass during their 

 development. 



The Recapitulation Theory explains this as indicating that 

 in these respects Amphioxus represents, more or less exactly, a 

 phase through which the higher Vertebrates have passed in the 

 history of their evolution ; that, as regards the organs in ques- 

 tion, Amphioxus may be viewed as figuring, with more or less 

 exactness, an ancestral form from which the higher types of 

 Vertebrates are descended. 



From this standpoint Amphioxus is an animal of very 

 special importance to morphologists ; and the development of' 

 Amphioxus acquires peculiar interest from the consideration 

 that, if the adult animal is far more primitive than any other 

 existing Vertebrate, then the earlier stages in its life history may 

 reasonably be expected, in accordance with the law of Recapi- 

 tulation, to yield valuable evidence as to the relations of Verte- 

 brates with the simpler groups of Metazoa. 



The above considerations do not imply that Amphioxus 

 itself stands in the direct line of ancestry of any of the higher 

 Vertebrates, but that it is a surviving representative of a type 

 of animals which preceded the higher Vertebrates in point of 

 time, and from which type, though not necessarily from Amphi- 

 oxus itself, the higher Vertebrates have arisen. 



Amphioxus shows us that, in attempting to reconstruct the 

 characters of the ancestors of Vertebrates, we are almost certainly 

 justified in omitting such features as paired limbs, a cartila- 

 ginous or bony skeleton, jaws, a twisted or chambered heart, a 

 highly specialised brain, and paired sense organs ; characters 

 which Amphioxus shows us are not necessary to an adult 

 Vertebrate, and in the absence of which the embryos of higher 

 Vertebrates agree with Amphioxus. 



A different explanation of the peculiarities of Amphioxus has 

 been offered by many zoologists, who consider that the simplicity 

 that characterises so many of its organs, as the brain, heart, 

 liver, &c., is not primitive, but due to degeneration ; that the 

 immediate ancestors of Amphioxus were, in fact, animals higher 

 in the zoological scale than itself. No distinct evidence of such 



