supporting lamella

518 W. B. HARDY.
different regions of those animals in such a way that the demands
of one part may be met by the discharge of stored
nutritive material from other regions. Such a fluid, which
Allmann has called the " somatic fluid," would not only contain
the immediate results of digestion, but also elaborated
material from the store of reserve nutriment possessed by
certain cells discharged in response to any special demand
in some particular region. In other words, it is not merely a
fluid for the distribution of the immediate products of digestion;
it is more than that, and we shall see reason to think that it is
a true metabolic link between one part of the body and another
strictly comparable to the blood of higher forms.
In Allmann's account of digestion in Hydroids he thus
describes the "somatic fluid :"—"Its basis is a transparent
colourless liquid, and in this solid bodies of various kinds are
suspended. These consist partly of disintegrated elements of
the food; partly of solid coloured matter which has been secreted
by the walls of the somatic cavity; partly of cells, some of
which have undoubtedly been detached from those walls,
though it is possible that others may have been primarily
developed in the fluid; and partly of minute irregular corpuscles,
which are possibly some of the effete elements of the
tissues."
Leaving the Ccelenterates and turning to the Turbellaria, we
have a striking instance of how the enteric cavity, the gut, may
serve as an organ for the distribution of nutriment. The case
of the Turbellaria also enables us to contrast cases 1 and
2, for in the Rhabdoccels we have animals with a simple gut
surrounded by a tissue, the mesenchyme, with numerous cleftlike
spaces, which may be so far developed as to form a fairly
well-marked space round the gut, as in Mesostoma tetragonum,
and to a less extent in Microstomum lineare.
That such a space or system of spaces facilitates the distribution
of material derived from the gut hardly needs stating,
whether we consider the distribution to be brought about by
diffusion or by the agitation of the contained fluid as a result
of the muscular movements of the animal.