REPORT ON SPIRULA. 
7 
Fig. a. 
Posterior part of 
the right side of the 
mantle in Sjiirula reti- 
culata, ventral view 
X 10. i, reticulation 
of the mantle ; ii, ter- 
minal cli.sk ; iii, shell. 
mantle that these spaces have the limits well marked. In Spirula reticulata, the 
reticulation is also more definite in this region as well as near the opening of the shell, 
where the polygons are more irregular (Fig. A in the text), whilst the projections of the 
reticulum vanish insensibly on the edges of these openings.^ 
From without inwards the mantle presents (besides the epithelium and the layer of 
chromatophores) : first, a layer of connective tissue, dense and glassy 
superficially, and looser below ; second, a thin layer of longitudinal 
muscular fibres ; third, a thick layer with circular muscular fibres, 
traversed by radiating muscular bundles (PI. VI. fig. 14) ; fourth, 
another thin layer of longitudinal muscular fibres ; and fifth, the 
subcutaneous connective tissue of the internal face. 
At the aboral part the form and the aspect of the mantle are 
modified by the presence of the shell, elsewhere almost entirely 
covered ; there is on both the dorsal and ventral faces only one 
longitudinal opening, in the form of an elongated ellipsoid, limited 
posteriorly by the “ terminal disk.” By these openings (the dorsal 
is the larger) the last whorl of the shell makes a slight prominence 
(PI. I. fig. 1, sh.). The external portions of the mantle are secondarily-acquired 
formations, which are almost totally wanting in Nautilus and most Gastropods. The 
mantle, properly so-called, is the little convex part of the wall of the shell cavity, 
that is to say, the envelope of the little visceral sac lodged in the terminal chamber of 
the shell (PI. III. P’^.) ; it is that envelope which is the true secreting surface of the shell. 
This little visceral sac corresponds to the visceral hernia of 
Nautilus and of the Gastropods, but does not include more, however, 
than the posterior half of the lobes of the liver (PI. III.) ; it is 
continued backwards by the membranous siphuncle (PI. V. fig. 1, 
sphm.) piercing all the septa of the shell, through the siphonal tubes 
of the shell {sph.) secreted by it. 
The proximal portion (quite anterior) of the siphuncle is much 
enlarged in Spirula reticulata (Fig. B, iv). At the level of the 
septum the siphuncle does not present any sensible constriction. The 
portion enclosed in the last chamber but one (consequently in the 
last segment of the shell-siphuncle) was surrounded, in the Spirula 
reticulata studied, by a thick muff of a hard substance, recalling by 
its aspect and consistency the ligament of certain Lamellibranchs ; this muff hermetically 
filled the space between the pallial siphuncle and the wall of the shell-siphuncle (fig. B, i). 
Fig. B. — Auterior part of 
the siphuncle of Spi- 
rula reticulata, left 
hand side view; magni- 
fied. i, mufi' of the last 
segment of the mem- 
branous siphuncle ; 
ii, membranous si- 
phuncle ; iii, shell- 
siphuncle ; iv, auterior 
widening of the si- 
phuncle ; V, mantle. 
1 Spirula “ examined by Owen in 1879 {Ann. Mag. Nat. Hist., ser. 5, vol. iii. pi. i. iig. 2), 
appears also to present a reticulation of the integuments ; this character will not then be special to S. 
“ reticulata.'’ 
