24 
THE VOYAGE OF H.M.S. CHALLENGER. 
all the Cephalopods, also merit the name of auricles ; they empty on each side into the 
ventricle (Fig. P ; PI. VI. fig. 8). 
Between the portion of the mantle enclosed in the last chamber of the shell and the 
part of the visceral sac contained therein, there is, in Spirula reticulata, a sharply 
limited sinus (Fig. Q, iv) with rather thick walls, communicating with the cavity of the 
membranous siphuncle ; no other communication has been seen 
with this sinus, but it is certainly to be presumed that it 
presents in some place a contractile orifice analogous to 
“ Keber’s valvule” in the Lamellibranchs. 
The physiological action of this sinus appears very 
important and apparently regulates the hydrostatic conditions 
of Spirilla, and consequently the production of new chambers 
of the shell. In fact the 
is a blood sinus continuous with the preceding (in the 
terminal enlargement of the siphuncle this cavity occupies the 
dorsal side, see Fig. Q). This membranous siphuncle may then 
be distended by the blood coming from the pallial sinus, and 
the constriction of its proximal portion by the hermetic muff of the shell siphuncle (Fig. 
B, i) permits it by its enlargement to compress the gas contained in the shell siphuncle, 
without this gas being able to flow back into the last chamber under the mass -of the 
liver. We thus explain how it can produce a change of equilibrium, in contracting or 
distending this gas, according as the Spirula wishes to descend or ascend, the pressure 
remaining always constant in the air chambers, quite separated from the siphuncle. 
On the other hand, w T hen by continued growth the weight of the animal threatens to 
become too great for the hydrostatic apparatus constituted by the air chambers of the 
shell, the distension of the pallio-siphonal sinus pushes forward insensibly the visceral 
mass resting upon the last septum, and thus permits the continuity of the secretion of the 
shell by the margins of the true mantle (PI. III., margins of P 7 ) ; the last chamber is 
thus completely formed. Then the contraction of the sinus clearing this last, a new 
septum is secreted in turn by the whole surface of the true mantle (PI. III., P 7 ) at the 
same time that a new segment of the shell siphuncle is produced by the membranous 
siphuncle. 1 
1 It is evident that a mechanical interpretation of the means of progression in the shell, and the ascent 
and descent in the water, is alone admissible, and that no naturalist will accept that proposed by Barrande in 
1877 : “They must there and then have been inspired and imposed by the Creator at the moment when the 
Cephalopods had been introduced among the inhabitants of the Silurian Seas ” (Barrande, Cephalopodes, 
Etudes generates, Prague, 1877, p. 210). 
cavity of the membranous siphuncle 
Fig. Q. —Median sagittal section of the 
terminal part of the siphuncle, left 
view; magnified, i, reflected mantle 
on the shell (ventralside)=P 8 , PL 
III.; ii, membranous siphuncle; iii, 
shell siphuncle ; iv, pallio-siphonal 
sinus ; v, penultimate chamber of 
the shell ; vi, last septum ; vii, 
mantle reflected on the shell (dorsal 
skle)=_P 5 , PL III.; viii, part of the 
visceral mass (liver) included in 
the last chamber. 
