118 MR. OWEN’S DESCRIPTIONS OF 
The affinity of the Argonauta to the Decapodous Cephalopods is further indicated by 
the inferior development of the mediastinal septum which divides longitudinally the 
branchial chamber. This septum in Eledone is complete and muscular throughout, ex- 
cepting a very small proportion of its inferior part. In the Octopus, in which this 
septum is well described and figured by Cuvier, as the ‘‘ bride antéricure qui lie la bourse 
a& la masse viscerale',” a greater proportion of the lower part is membranous than in 
Eledone. In the Argonauta the muscular part of the septwm is reduced to two narrow 
and delicate strips, which arise from the posterior part of the cranial cartilage, descend 
obliquely forwards, intercept the termination of the rectum and ink-passage, to which 
they serve as a sphincter, and then expand in the vertical direction, to be inserted along 
the middle line of the internal surface of the anterior part of the mantle : a membrane 
is continued from their upper part to within a short distance of the margin of the mantle ; 
and another from their lower part extends downwards, and terminates opposite the base 
of the gills ; the branchial chambers intercommunicate both above and below this sep- 
tum. In Sepiola the muscles corresponding to the bride antérieure of the Octopus are 
developed in the same degree as in the Argonauta ; but the membranous part of the 
septum above them is wanting, while that which is continued from their inferior mar- 
gins is more complete. In the Calamaries both these muscles and the septum of the 
branchial chamber are wanting. 
With respect to the nervous system of the Argonauta, I find in a large specimen of 
the Argo, that the brain, when viewed from the superior or dorsal aspect, presents, as in 
Octopus, an anterior, white, flattened, tranversely oblong band, and a posterior raised 
convex semilunar mass, which terminates by a semicircular border posteriorly, the ex- 
tremities of which are directly continued, to form or join the posterior nervous collar 
of the wsophagus. The great lateral nerves of the mantle come off from the posterior 
subcesophageal mass, precisely as in Octopus ; and instead of extending down in a pa- 
rallel direction as low as the roots of the gills, as represented in the splendid figure by 
Delle Chiaie', they diverge, penetrate the short muscles, analogous to the brides late- 
rales in the Poulp, and terminate in the stellated ganglions opposite the upper extre- 
mities of the gills, and immediately below the base of the funnel : by some unaccountable 
error, these nerves, in Delle Chiaie’s figure, which some of our compilers have copied, 
are made to come off from the optic ganglions. The lateral muscles above mentioned 
are the analogues of the great shell-muscles of the Nautilus Pompilius ; they are more 
strongly developed in Loligo and Sepia than in Octopus, but have the same origin in 
each, the same attachment to the capsule containing the rudimental shell, and are al- 
ways perforated by the great lateral nerves of the mantle: they are perforated by the 
corresponding divided and ungangliated nerves in the Nautilus, in which these mus- 
cles acquire the mawimum of development. In the Argonauta, which has no muscular 
attachment to its shell, and has no internal festaceous rudiment, the corresponding 
' Poli, Testacea Utriusque Sicilia, vol. iii. pars 1™, posthuma, 1826. 
