S THE VOYAGE OF H.M.S. CHALLENGER. 
Tlie surface of tlic little visceral sac (the mautle consequently) is reflected above the 
ojHJiiing of the shell, pas.scs into the wall of the “ episoma ” clcsely attached to it 
(separated bv a space in PI. V. fig. l), and evidently secreting (upon the median line) the 
“Kind of thickening” (PI. II. fig. 7, h). The mantle thus forms a shell cavity, closed 
on all sides e.xcept at two opposite points, the dorsal and ventral openings of the shell, 
by which the portions of the la.st whorl of the shell make a free projection outside. 
These shell openings, in short, form an absolutely natural disposition, although in 
the Challenger s]>ecimen, where the integuments of this region were considerably altered, 
the margins of the openings appeared to be fixed and to have thus sent short 
irregularly-cut prolongations over the shell (PI. I. 
figs. 1, 3); in Spirula australis {V\. TI. figs. 1, 2), 
and in Spirula reticulata (Fig. C in the text), this 
margin is perfectly complete and uniform, and there 
is no portion of the integument, however thin this 
may be, which passes over the shell, contrary to the 
opinion of Steenstrup^ and Owen, according to whom 
the periostracum (shell epidermis) was continuous with 
the epithelium of the margin of the pallial openings.^ 
As one might expect, this last assertion is absolutely 
incorrect. This periostracum is continuous under the 
mantle over the whole surface of the internal portions 
of the shell, and it is in no way a cellular tissue. 
In front and upon the sides, the wall of the shell 
sac is formed by the thin “ peritoneal ” membrane, 
which constitutes the external envelope of the viscero- 
pericardial cavity (completely separated from the 
„ shell sac) ; posteriorly it is formed by the terminal 
t'W- < — SptruJa rrticuiata, with the (lalluil cavity ' ^ . . . 
UM open, vrnirai view; X 4. 1 ventro-iatcrai disk. Internally the shell sac is limited by the little 
of the edjfe of the mantle ; li, gill ; iii, •' •' 
mant]c, properly so-called (see above), and by its 
continuation, the siphuncle (PI. V. fig. 1, sjohm.), 
ai. aonai , ejection of the mantle eiige. extending as far as the first chamber. 
1 pon the wall of the shell sac are inserted laterally : in front, the retractor muscles of 
the branchiae (which on parting from the branchia are at first transverse, then directed 
|»o«tcriorly) ; l>chind, the muscular bundles of the fins. 
h. Tvrmtnnl Jhsk. — The terminal disk is fleshy and covers the most aboral parts of 
the shell with a thick layer of tissue limited all round by a superficial furrow more or less 
• Htectutnip, rrjt, ri/., j». 227 : " Shell rliutinctly covered dorsally and ventrally, where the skin grew 
thin atioTc it” 
• f>w«*n, ytm, Mao. Sat. UiM,, ser. 5, vol. iii. p. 3. 
