14 
THE VOYAGE OF H.M.S. CHALLENGER. 
not present siphonal necks, but each segment of the siphuncle is joined to the subsequent 
one (that which is formed after it) by a sort of “ cogwheel,” more or less regular, which is 
found at the aboral extremity of the former, spreading a little over the concave surface of 
the septa (PI. II. figs. 4 and 6), and thus preventing the penultimate chamber from com¬ 
municating with the siphonal cavity. 1 The apical extremity of the siphuncle is a caecum 
making a small prominence in the interior of the initial chamber (PI. II. fig. 7, psph.) ; 
from this caecum a sort of ligament, situated in the median sagittal plane, runs, while 
enlarging, to the wall of the initial chamber (prosiphonal ligament or “ prosiphuncle,” 
PI. II. fig. 7, c.). We can only make conjectures as to the nature and function 
of this “ ligament.” But it evidently cannot have had the function of siphuncle in the 
embryo ; 2 the embryo with a single chamber cannot have had any kind of siphuncle, 
and from the formation of the second chamber the true siphuncle was constituted. 
The fact that in our Spirula reticulata the last chamber is so shallow renders it 
probable that the increase of the shell-siphon is intermittent, and that each segment of the 
siphuncle does not originate much before the septum which is topographically anterior to it. 
d. Pallia1 Cavity. — The pallial cavity is widely open, the margin of the mantle not being 
at any point united to the cephalopedal mass. But it is considerably less deep than in the 
other Dibranchiates ; the presence of the shell prevents its extension on the median line (PI. 
I. fig. 6 ; Pis. III. and IV.), and even on the sides it does not extend very much further 
(PI. IV. figs. 1 and 3, a). In Nautilus likewise the pallial cavity does not extend so far 
backwards as in the other Cephalopods. Upon the “ visceral ” face of the pallial cavity, 
at the very posterior extremity, are found (in the female) the nidamental glands (PI. I. fig. 
6, n.g., &c.). In front, and partially concealed by the anterior extremity of these glands, 
are glandular prominences (PI. I. fig. 7, x ; Fig. C in the text, iii), apparently accessory 
nidamental glands (see further on, Reproductive System); the left one partly hides the 
extremity of the oviduct (Fig. C, ix). In Spirula peronii and Spirula australis (PI. I. 
fig. 7 ; PI. II. fig. 1) the anus opens between these two prominences on the level of 
their anterior margins, whilst in Spirula reticulata it, as well as the two neighbouring 
sessile renal openings (Fig. R), is hidden on the median line behind these two prominences, 
which are almost in contact; between the two prominences there is seen, beneath the 
integuments, the ink bag (Figs. C and R, iii). The glandular extremity of the oviduct 
projects between the left gill and the glandular prominence, behind which it sinks and 
opens obliquely (Fig. R, vii). 
The gills are situated quite laterally (PI. IV. fig. 3, bd), less ventrally than in other 
Cephalopods. They are relatively small, and present about twenty-five pairs of leaflets. 
1 The remarkable radiating structure of the siphuncle of certain palaeozoic Cephalopods is perhaps 
comparable to it. 
2 Munier-Ch almas, Sur le developpement du phragmostracum des Cephalopodes et sur les rapports 
zoologiques des Ammonites avec les Spirules ( Comjptes rendus, t. lxxvii., 1873, p. 1559). 
