THE VOYAGE OF H.M.S. CHALLENGER. 
(> 
the iuternal face of tlie mautle (PI. II. hg. 1, Ipa.), which form with these the resisting 
ap|>aratus destined to insure the closure of the pallial chamber while the water is 
e.\|)elled through the funnel. The nuchal cartilage (PI. I. fig. 5, d. i.) on the anterior 
ilorsal margin of the “ collar ” in the median line has nearly the form of that in Loligo, 
and especially in the Ommatostrephidm. 
The retractor muscles of the siphuncle (PI. IV. l.mr.) and of the head unite laterally, 
and have their common origin upon the external sides of the last chamber of the shell. 
The retractile })art of the head of these latter muscles is inserted upon the aboral 
face of the perineural or cephalic cartilage, which forms a capsule enveloping the nervous 
centres. This cartilage sends, upon the sides, vast prong-like prolongations (PI. V. fig. 3, 
pn.ct*.) supporting the optic ganglia, and ventrally, in front, the median projection 
(PI. V. fig. 3. pyi.c^'.), which (as in Sepia) gives attachment to the muscles of the arms. 
In front, the cartilaginous capsule is continued by the envelope of connective tissue 
of the junction between the pedal and brachial centres. 
4. Mantle. — «. Pallial Envelope. — The external surface of the mantle is smooth, as 
in the other Dibranchiates ; the reticulation observed in the form called Spirula reticulata 
does not belong to the superficial portion of the integuments, as will be seen later on. 
The border of the mantle, free all round its circumference, offers three anterior 
protuberances : a median dorsal one, and two symmetrical latero- ventral ones, which 
limit the " sinus ” of the funnel (PI. I. figs. 2, 3 ; and Fig. C). 
In the Challenger specimen the external surface of the mantle was deprived of its 
cj)idermis, and, for the greater part of its extent, of the layer of chromatophores.^ The 
chromatophorcs, however, were present at the aboral extremity (PI. VI. fig. 14), where the 
skin was not removed, as well as upon a narrow band of the internal face of the mantle 
along the margin at this place. The “ Blake ” specimen was in this respect in the same 
condition, the chromatophorcs being preserved only at the aboral extremity (PI. II. figs. 
1, 2) and along the margin of the mantle on the internal face (PI. II. figs. 1, 2, cr.). 
The chromatophorcs are nevertheless spread over the whole external surface of the 
mantle.’ In fact there exists in the form called Spirula reticulata a general reticu- 
lation of this surface given as a character of that form, and due, from what I have seen, 
to bundles of subcutaneous fibres disposed in such a manner as to form more or less 
regular |»f»lygons. 
However, these polygonal spaces with definite limits exist also in an Ommatostrephid, 
lilej- c' .utleti, where on taking away the pallial epithelium, an appearance is observed 
v»-ry similar to that jtresented by Sjnrula reticulata, and the polygonal spaces arc there 
iM-rupicd by chromatophorcs. In Illex it is especially near the free margin of the 
' It w prolmhly thuttatc whicli led von ^Villenloc«-Suhra to suppose that the specimen had been swallowed 
V '7 an «b}TMal fi*h and r<*gurgilaUxl when brought to the surface by the trawl. 
* Tl)*’ '■oloration of the dea<l •pccinicns, but freshly taken from the water, is yellowish-white spotted with 
l-n.wD. arcordjng It/jbi rt {Omptrj, retuhu, t ii., 1836, p. 3G2). 
