558 WILLIAM BATESON. 
fact does not interfere with the obvious possibility of a digging 
mouth having again intervened, from which such a mouth as 
that of the Lampreys could easily be derived. 
Taking into consideration, then, the fact that in the most 
primitive forms the mouth is anteriorly directed, and that in 
the Lampreys it is also anteriorly directed, though of different 
function, we may tentatively suppose that though the mouth of 
the possibly original pelagic form was directed ventralwards, and 
was possibly suctorial, yet probably the mouth of the Marsipo- 
branchs is derived from a digging ancestor, in which the mouth 
of the hypothetical pelagic form had come to be anteriorly 
directed in correlation with an acquired burrowing habit. In 
any case the facts of the Enteropneusta entirely confirm Bal- 
four’s view, that the Vertebrate jaws have been developed com- 
paratively Jong afterwards. 
The Skin.—That the skin was originally ciliated there can 
be little doubt; also it is probable that at first plexuses of 
nerve-fibre were formed at the base of the ectoderm cells, such 
as may be seen in many if not in all animals with ciliated skins 
of this type. 
The Nervous System.—The next question relates to the 
position and mode of the first formation of a differentiated ner- 
vous system. The evidence of Enteropneusta, Ascidians, and 
Amphioxus is united in showing that this first occurred in the 
dorsal middle line, and not by the coalescence of two lateral 
cords. The structure of the nervous system of Balanoglossus 
further shows us a stage in the process by which this nervous 
cord separated from the skin. By many authors it is supposed 
that this was accomplished in the first Chordata by an invagi- 
nation, but the evidence of Balanoglossus is decidedly for the 
view that a process of delamination preceded this ; and, indeed, 
this being the simple process, might naturally have been ex- 
pected to have occurred first. In Balanoglossus we seein 
the trunk the cord still in the skin, in the collarthe 
cord delaminated, and at the ends of this cord the 
process of invagination commencing and leading to 
the presence of a lumen, More than this, the mode of 
