•24 
THE VOYAGE OF II.M.S. CHALLENGER. 
Kto. Q.— u^ttal section of the 
tcnnin&l |«rt of the siphuucle, left 
ms^ifietL i, reflrcte«l mnntle 
on the shell (ventral Hi<ie) = i*', PI. 
III.; ii. membranous sinhuncle; iii, 
•hell siphuncle ; ir, |allio-siphonal 
sions ; V, {lenultiroatc chamber of 
the shell ; vi, last sentuni ; vii, 
mantle retlectccl on the shell (dorsal 
!!ye) = /*, PL 111.; viiL part of the 
visceral man (liver) included in 
the last chamber. 
all the Cephalopotls, also merit the name of auricles ; they empty on each side into the 
ventricle (Fig. P ; PI. VI. fig. 8). 
Bet ween 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 cavity of the membranous siphuncle 
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 muflf of the shell siphuncle (Fig. 
B, i) permits it by its enlargement to compress the gas contained in the shell siphuncle, 
without this ga.s 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 
•Ustending 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, when by continued growth the weight of the animal threatens to 
l)crf»mc too great for the hydrostatic apparatus constituted by the air chambers of the 
shell, the di.stension of the pallio-siphonal sinus pushes forward insensibly the visceral 
inaj-.-i rc.sting 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^) ; 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^) at the 
=ame time that a new segment of the shell siphuncle is produced by the membranous 
siphuncle.' 
‘ It ij --viih.nt that a mechanical interiircUition of the means of progression in the shell, and the ascent 
1 <h • nl in the waUr, U alone admissible, and that no naturalist will accept that proposed by Barrande in 
ls77 : “They must there and then have l»cen inspired and imposed by the Creator at the moment when the 
C- :phslo|«da hail bei*n intrrsluced among the inhabitants of the Silurian Seas ” (Barrande, Cciphalopodes, 
g:.*; irjia, I’mgue, 1877, p. lilO). 
