6 
THE VOYAGE OF H.M.S. CHALLENGER. 
the internal face of the mantle (PL II. fig. 1 , Ipa.), which form with these the resisting 
apparatus destined to insure the closure of the pallial chamber while the water is 
expelled through the funnel. The nuchal cartilage (PI. I. fig. 5 , cl. i.) on the anterior 
dorsal margin of the “ collar ” in the median line has nearly the form of that in Loligo, 
and especially in the Ommatostrephidse. 
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 part 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 1 .) supporting the optic ganglia, and ventrally, in front, the median projection 
(PI. "\ . fig. 3 , pn.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. — a, Pallial Envelope .—The external surface of the mantle is smooth, as 
in the other Dibranchiates ; the reticulation observed in the form called Spirilla 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 
epidermis, and, for the greater part of its extent, of the layer of chromatophores. 1 The 
chromatophores, 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 chromatophores 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 chromatophores 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 polygons. 
However, these polygonal spaces with definite limits exist also in an Ommatostrephid, 
Illex coindeti, where on taking away the pallial epithelium, an appearance is observed 
very similar to that presented by Spirula reticulata, and the polygonal spaces are there 
occupied by chromatophores. In Plex it is especially near the free margin of the 
It is probably this state which led von V illemoes-Suhm to suppose that the specimen had been swallowed 
by an abyssal fish and regurgitated when brought to the surface by the trawl. 
The coloration of the dead specimens, but freshly taken from the water, is yellowish-white spotted with 
brown, according to Robert (Comptes rendus, t, ii., 1836, p. 362). 
