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K. M. SMITH AND H. GL NEWTH. 
suggests, upon tlie amount of fluid contained in the cavities, 
but the actual ectodermal profile we take to be an artifact. 
On the left side, where the ectoderm is anchored, so to speak 
— by its fusion with the gut, to form the mouth, and with the 
head cavity to form the pree-oral pit — the swelling occurs to 
a lesser degree than on the right side, where no such attach- 
ments exist; and in the trunk region, where the cavities of 
the myotonies are small or virtual, and the close apposition of 
the walls of the splanchnocoel would preclude osmotic disrup- 
tion, it occurs not at all. The artificial nature of the disten- 
sion in question is frequently made apparent by a separation 
of the extremely thin outer wall of the somite from the 
ectoderm. This is seen even in so young a larva as that 
shown in PI. 18, fig. 1 (q. v.) ; in later larvse the action of the 
fixative is often to rupture the ectoderm on the right side. 
The communication between the left collar cavity and the 
gut, described by MacBride, is indicated in larvge of this 
stage (PI. 18, figs. 2 and 3). It still appears in the majority 
as a funnel-shaped depression in the dorso-lateral wall of the 
gut, with sometimes a plug-like fascicle of columnar cells 
filling it (PI. 18, fig. 2). It is never, in our sections, a very 
definite structure, but it gives a characteristic shape to the 
lumen of the gut, which is generally found to persist through 
several sections. In the preparations we have examined of a 
slightly later stage (mouth and one gill established) we have 
failed to demonstrate its presence ; but the swollen, vacuo- 
lated condition of the gut cells in this later stage makes it 
easily possible that the connection, though not recognisable, 
is present. 
In later larvae still (seven or eight primary gills present) 
the nephridium of Hatschek appears, as described by 
Goodrich (6), lying in a space which is apparently a backward 
prolongation of the left collar myocoel. But whether this 
space is the original portal of communication, drawn out into 
a tube by the growth of the larva, and, if so, how the 
nephridium (an organ presumably of ectodermal origin) 
comes to lie naked within it, are questions that our materials 
