INTERNAL ANATOMY. 51 



J. W. VAN WIJHE, it would appear that this so-called aortic 

 arch does not communicate with the branchial artery, but 

 ends blindly below in the neighbourhood of the right meta- 

 pleur. Dorsally, the aorta from which this lateral arch-like 

 outgrowth occurs, is continued forwards (not as a simple 

 vessel, but as a complex of vessels) as far as a peculiar 

 sense-organ known as the groove of Hatschek, after its 

 discoverer. This groove lies in the roof of the oral hood 

 to the right of the notochord, and is derived from the 

 pr&oral pit of the larva (see below). (Cf. Fig. 76.) 



In front of the sense-organ this dilated continuation of 

 the right aorta communicates beneath the notochord by 

 means of a transverse vascular commissure with the left 

 aorta, which retains its small calibre and simple character 

 throughout. From the vascular complex of the right 

 aorta arise the vessels which supply the buccal cirri. 



Hitherto we have only spoken of those blood-vessels 

 which are related to some part or other of the alimentary 

 canal. In point of fact the parietal or somatic vessels of 

 Amphioxus, if present at all, must have a very subordi- 

 nate physiological significance. Their place is taken by 

 lymph-spaces, of which there are a great number in various 

 parts of the body. Such are the dorsal and ventral fin- 

 chambers, the spaces in the metapleural folds, spaces at 

 the apices of the myotomes and in connexion with the 

 dorsal nerve-roots, etc. (Cf. Fig. 2.) 3 



The vascular system of Amphioxus presents several 

 features of great interest from a phylogenetic or evolu- 

 tionary point of view. 



We have seen that the heart is in no way differentiated 

 from the branchial artery and is therefore a simple tubular 

 vessel. This is the primary condition of the heart in the 

 embryos of all the craniate Vertebrates. In the latter, as 



