THE FOEMATION OP THE PAYEES IN AMPHIOXUS. 337 
the edge of the oral hood in Ainphioxus occupy a corres- 
ponding position to the “ arms” of Cephalodiscns, and are 
in all probability homologous therewith. Adding the “arms” 
to the collar of a Balanoglossus and shortening the worm- 
like trunk, the length of which is a result of burrowing' life, 
we arrive at a conception of the primitive Vertebrate stock 
to which the free-swimming ancestors of Echinodermata were 
allied. In the Echinodermata the fixed habit was adopted 
and one of the “collar-cavities” gretv at the expense of the 
other and so the water-vascular ring was formed. 
Professor llubrecht indicates a belief that the “supposed 
Actinian ancestor developed into a vermiform one.” What 
Text-fig. 10. 
The foimuon ancestor of Vertebrata, Enteropneiista and 
Echinodermata. 
kind of a “vermiform” ancestor Hubrecht means he does 
not indicate in the paper under discussion. To judge from 
his previous work one would suppose that the “ worm ” was 
a Nemertean. The theory of the Nemertean origin of Verte- 
brata was published in this Journal in two editions. In the 
later and revised edition (17) we learn that the Vertebrate 
nervous system corresponds to an inconspicuous dorsal nerve 
in the Nemertinea, whilst the main nervous system of those 
“worms” gives rise to the chain of cranial ganglia. The 
proboscis of Nemertinea becomes the hypopliysis of Verte- 
brata, whilst the proboscis-sheath forms the notochord. The 
question of the coelom is passed over as a difficult and obscure 
(piestion. As, however, the figure he gives of the vermiform 
ancestor in the present paper is adorned with a number of 
