II. 



ANATOMY OF AMPHIOXUS. 



INTERNAL ANATOMY (continued). 



IN the preceding chapter we have seen how Amphioxus, 

 while possessing the general facies of a fish, and the 

 primary essential attributes of a Vertebrate, is nevertheless 

 destitute of many of the most obvious structural features 

 which we usually associate with our conception of a fish. 

 Thus it has no skull, or, in other words, it is Acmniate 

 (Haeckel). It has no jaws, and is therefore a Cyclostome, 

 as opposed to a Gnathostome. Finally, it has no paired 

 sense-organs and no paired muscular fins. Its eye-spot 

 is median, like that of a Cyclopean monster. There is no 

 trace of an auditory organ of any kind, while the single 

 so-called olfactory pit, abutting on the anterior end of the 

 nerve-tube, has been regarded as an indication of a mono- 

 rhinic condition preceding the amphirhinic, i.e. with paired 

 nostrils. 



Vascular System. 



Now, in turning our attention to the vascular system, we 

 shall find that Amphioxus has no heart. In any ani- 

 mal with a comparatively well-developed vascular system, 

 the presence of a heart might be regarded as a sine qua 

 non. This, however, is by no means always the case ; and 

 although, among the Invertebrates, the extensive groups 



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