W. K. Brooks— Cephalopod Siphon and Arms. 289 
Figure 1 is a squid embryo with its head and yolk sae, Y, 
below, and the so-called posterior end above; the so-called 
ventral surface, which in the adult carries the siphon, at the 
left, and the so-called dorsal surface at the right. 
Figure 2 is the embryo of a fresh-water pulmonate in a sim- 
ilar position : with the foot, F, below ; the shell area, SA, above ; 
the mouth, M, tentacles, T, ‘and head on the right side, and 
the rectum, R, on the left. The figure therefore Snaws the 
right side of the pulmonate. 
SH is the bilaterally symmetrical shell, resting like a cap 
upon the dorsal surface of the body, and surrounded by a 
reflected ridge of integument, SA, the margin of the shell area. 
In the squid, figure 1, there is also an external shell which 
is surrounded by a reflected ridge of integument, SA, as in 
the pulmonate. 
Running around the shell and shell area, in both, is a second 
ridge of integument, the margin of the mantle, MA, which, in 
the cephalopod, already projects a ee forming the outer 
wall of the rudimentary mantle chamber 
On the median line of the posterior, or ventral, surface of 
the body, just below the mantle ridge, in cae is the rectum, 
R, with its opening the anus. 
‘In the en the anus is raised from the surface of the 
y upon an anal papilla, but in other respects it is alike in 
the two forms. 
On each side of the rectum of the cephalopod is a tentacular 
gill, G, prhcreonun ese projecting edge of the dees There. 
are oe correspon structures in the pulmo 
wiadichion ‘mites of the body, the georee which is 
pre alla dorsal in the cephalopod, and which is on the 
Am. Jour. Sct. tle’ — Von, XX, No, 118.—Ocr., 1880. 
