290 W. K. Brooks—Cephalopod Siphon and Arms. 
right in both figures, we have, first the thickened mantle ridge, 
MA, which forms the angle between the dorsal and the ante- 
rior surface ; next we have the long pulsatile “neck” region of 
the pulmonate, H. In the cephalopod embryo this region is 
short, not pulsatile, and forms the back. 
Below the ‘“‘neck” of the pulmonate a tentacle, T, is devel- 
oped on each side of the body, and the eyes subsequently 
make their appearance upon these tentacles. 
On each side of the corresponding region of the body of the 
cephalopod embryo there is a projecting eye-stalk, T’, upon 
the rounded tip of which the eye is formed by an involution 
of the integument. 
Crossing the middle line of the body of the pulmonate just 
below the tentacles, is the rudimentary velum, V, which runs 
out onto the sides and then bends up towards the dorsal surface 
in such a way as to almost surround the tentacles. This line 
is marked in the pulmonate by a row of granular ciliated cells. 
When the corresponding surface of the body of the cepha- 
lopod is examined, a well defined line or groove, V, figure 1, 
will be found to run from the median line out into the sides of 
the body, bending ap posteriorly in such a way as to nearly 
surround the eye-stalk. The i 
e have then the following structures which are so similar 
in position, relations, mode of development, and function, as to 
leave no doubt of their homology ; the mouth ; the velum; the 
sensory tentacles; the mantle; the shell area; the shell; the 
rectum and the anus. 
These features furnish enough points of orientation to assure 
us that the cephalopod must be placed as it is in our figure 1, 
in order to be in a position homologous with that of the gaster- 
opod embryo, figure 2. e views which have been advoca- 
ted by Huxley are therefore essentially correct. 
In order to show the relation between figure 1 and an adult 
cephalopod, I give, in figure 3, an outline sketch, much less 
magnified, of an older embryo which has acquired most of the 
adult characteristics. As the position of this figure is like that 
of the other two the dorsal surface is that which is above, the 
ventral is below, the anterior surface on the right, the posterior 
on the left, and the figure shows the left side of the animal. 
We are now in a position to discuss the disputed points in 
cephalopod structures; the homology of the siphon, and the 
arms ; and the equivalent of the gasteropod foot. 
