118 MR. OWEN'S DESCRIPTIONS OF 



The affinity of the Argonauta to the Decapodous Cephalopods is further indicated hy 

 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 byCuvier, as the " hride anterieure qui lie la bourse 

 a la masse visce'rale\" a greater proportion of the lower part is membranous than in 

 Eledone. In the Argonauta the muscular part of the septum 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 biide anterieure 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 oesophagus. The great lateral nerves of the mantle come olF from the posterior 

 suboesophageal 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, whicli 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 maximum of development. In the Argonauta, which has no muscular 

 attachment to its shell, and has no internal testaceous rudiment, the corresponding 



' Poli, Testacea Utriusque SicUice, vol. iii. pars l™^ posthuma, 1826. 



