THE FORMATION OF THE LAYERS IN AMRHIOXUS. 335 
from in front backwards and the anus is the only remnant of 
it; the present mouth is a new formation. In this theory 
Hubrecht supposes that he is reviving an old theory of 
Sedgwick’s as to the origin of the Metazoa (30). But on 
Hubrecht’s theory the coelom is of ectodermal origin, and 
must have originated from stomodmal pockets which do not 
exist in Actinozoa, whilst Sedgwick regards the coelom as 
derived from the inter-mesenteric snaces of the true endo- 
X 
dermal gut. Sedgwick’s theory was published about a quarter 
of a century ago, and was based on a review of all the 
evidence available at the time. It is a theory of the origin 
not of Vertebrata but of all Metazoa, and it traces them back 
to an Actinia-like ancestor. The theory of Sedgwick may be 
analysed into three parts — for the name Actinia given to the 
common ancestor was, of course, only an indication of a very 
general resemblance to a modern sea-anemone. The three 
parts are: (1) The position that the coelom arose as pockets 
of the original gut or archenteron ; (2) the position that 
mouth and anus are two separated portions of a long slit-like 
mouth ; and (3) the idea that the central nervous system is 
homologous throughout the whole of the Metazoa and had 
originally the form of a ring round the original mouth. Now 
I venture to maintain that the first two parts of the theory 
have received more and more support as embryological 
research has gone on, but that the third part must be given 
u]) and that the comtnon Metazoan ancestor in consequence 
takes on more resemblance to a Ctenophore than to an 
Ac ti Ilian, for it has become evident that in many types of 
larva) the apical plate of neuro-epithelial cells is the first 
rudiment of the brain, and that it is independent of post-oral 
nervous aggregations. ’Jliat the coelom arises in Amphioxus 
as five archenteric outgrowths I trust I have convinced the 
readers of this paper. ’J'hat the mouth and anus in all 
animals in which two such openings are found, owe their 
origin to the division of a long slit-like cceleuterate mouth I 
firmly believe. If, however, the affinities of Amphioxus 
are to be sought for in the vicinity of Balanoglossus, then 
