RECENT ADVANCES IN ZOOLOGY. 291 
orifices by which the canals of our common Calcareous Sponge 
(Grantia compressa) communicate with the exterior are fringed 
with delicate hair-like processes which taper to a fine point; each 
hair appears to be in relation to an underlying cell, which sends 
out a delicate process which traverses the axis of the hair-like 
process. This apparatus is judged by Prof. Stewart to be one 
which is specially adapted for being impressed by varying condi- 
tions in the inrushing water; these changes possibly lead to the 
contraction of the surrounding cells, so that we have here a 
mechanism by means of which the extent of the incurrent canal 
may be increased or diminished. 
In the next group of animals we have a number of stages in 
the development of sense-organs, and a number of jelly-fish are, 
without doubt, provided with means by which they can distinguish 
between light and darkness. But there are not only sense-organs 
situated on the surface; there is, also, just below the under 
surface of the disc or umbrella, a network of nerve-fibres which 
extends in an apparently irregular fashion through its whole 
extent. In other words, when a nervous system first appears it 
is superficial in position and scattered in arrangement. 
As the nerve-fibres become more and more concentrated they 
become arranged in a smaller number of definite bundles, which 
we are in the habit of calling nerve-cords. Of these there may 
be, as Gaffron has lately found in one of the Flukes (Distomum 
isostomum), six cords, a pair of which are dorsal, a pair of which 
are lateral, and a pair of which are ventral in position. Now, of 
these, it is quite clear that future circumstances would alone 
determine which were to gain the supremacy, and become the 
dominant pair; to put it in another way, such a fluke might have 
three sets of descendants in which a dorsal, a lateral, or a ventral 
pair would alone be found. 
The Nemertinea carry us a step further in our argument; 
while Carinella has its nerve-cord lying outside the muscular 
wall of the body and just beneath the integument, Cerebratulus 
has its nerve-cords lying in the midst of its muscular walls, and 
Amphiporus has them internal to the muscles; but in all these 
three cases the cords lie midway between the dorsal and the 
ventral surface of the body; they are lateral in position. 
In most ‘Invertebrates’ the cords finally take up a ventral 
position ; but this is not always altogether the case, 
