NATURAL HISTORY OF THE GENUS DERO. 
101 
of this genus for nearly two years, have examined hundreds of 
examples of its various species, and have done my utmost to make 
sure of every fact which I advauce. It is true that in three 
species supplementary branchiae are present, but these are 
much smaller than the primary ones, and are always placed at 
the angles of the dorsal lip of the area. Where more than three 
pairs are attributed to any species, the incurved margins of the 
area, which are ciliated on both surfaces, have been mistaken 
for additional branchiae. 
The integumental and mucous layers of the area are not 
equally extensive ; the mucous layer is wanting (PI. IY. fig. 5) at 
the tip, and encroaches on the margin of the integumental layer 
at every other part. The branchiae are simple elevations of the 
mucous layer, containing loops of blood-vessels one in each, and 
are lined by a layer of stellate muscle-cells continuous with the 
layer which underlies the mucous membrane of all the other 
portions of the area. This muscular network is capable of great 
expansion and contraction, and in the latter condition fully 
realizes Bose’s description of Nais ciuricularis. The area is 
extremely sensitive, the slightest jar causing contraction, and, 
what is somewhat remarkable, it appears to be endowed with 
sensibility to actinic light # . 
In some species, in addition to the muscular network of stel- 
late cells already spoken of, an arrangement of radiating cells, 
spindle-shaped, with forked extremities and a central nucleus, is 
found connecting the dorsal lip of the branchial area with the 
intestine beneath it. 
In most cases the branchial area is concave in full dilatation ; 
but in Dero Mulleri it is capable of such expansion as to become 
altogether convex, and to stand at right angles to the axis of the 
body. The same is the case to a less extent in D. latissima. 
The chief blood-vessels are, as usual, two in number, a dorsal 
(arterial) and a ventral (venous). The dorsal vessel in Dero can 
* I have several times tried to photograph the tail of Dero, but hitherto 
with small success, the ignition of the magnesium-ribbon employed as 
the source of light causing contraction in nearly every case. The same phe- 
nomenon occurs in Hydra. In order to overcome this difficulty a drop-shutter 
was placed between the source of the light and the object, but the result was the 
same ; and the marvellous rapidity of the contraction was shown by the fact 
that, although fully expanded up to the instant of exposure, the photograph in 
every case showed the polyp in a contracted condition. 
