VOL. III.] Balanoglossus. 191 



cartilaginous sheath, which is directly continous with a corresponding 

 sheath surrounding the notochord, while in Balanoglossus no such 

 sheath is found, the nerve cord and notochord not only not being 

 in close relation, but the dorsal blood vessel is situated between 

 them. However, so far as the absence of the sheath is concerned, 

 the difficulty is hardly a weighty one since we must suppose, both 

 from developmental evidence and on a priori grounds, that the 

 earliest vertebrate ancestors were without such a sheath. But the 

 situation of the dorsal blood vessel as described is not so easily ex- 

 plained away, though Dr. Morgan* has suggested that the dorsal 

 aorta ol vertebrates is another vessel entirely. His suggestion 

 would seem to imply that in vertebrates the dorsal aorta has arisen 

 since the vertebrate phylum branched off from the common ances- 

 tral form, and that the dorsal vessel corresponding to the one now 

 found in Balanoglossus has disappeared. This conjecture may re- 

 ceive support from the fact that the heart of Balanoglossus is situated 

 in the proboscis, and hence cannot certainly have any relation to the 

 vertebrate heart. 



In this connection it seems to me worth while to refer to the 

 lymph canals described by Lankester^ within the notochordal 

 sheath, one on the dorsal side and one on the ventral side, in Am- 

 phioxus. And the same author speaks of the great difficulty in 

 distinguishing blood vessels from lymph vessels in this animal. It 

 would be rash to maintain a homology between the lymph canal in 

 the dorsal portion of the Amphioxus notochord and the dorsal blood 

 vessel of Balanoglossus, yet no harm can come from a cautious sug- 

 gestion of such a possibility. 



The notochord of Balanoglossus originates from the dorsal wall 

 of the digestiv^e tube as it does in vertebrates, and in later stages ot 

 development resembles the vertebrate notochord in its histological 

 structure considerably, thus satisfying two of the important criteria 

 of homologous structures. But, in all vertebrates, without excep- 

 tion, the notochord arises from nearly or quite the entire length 

 of the embryonic digestive tube, while in Balanoglossus it arises as 

 an evagination from near the anterior end and grows out anteriorly 



^ T. H. Morgan. Growth and Metamorphosis of Tornaria, Journ. of Morphology, 

 Vol. V, p. 407, 1892. 



^ E. Ray Lankester. Contributions to the knowledge of Amphioxus lanceola- 

 tus, Yarrell, Quart. Journ. Micro. Sci., Vol. xxix, p. 365, 1889. 



