340 LECTURE XXIV. 



exception the mantle is naked, and includes a more or less rudimental 

 shell ; and it supports, in most of the species, a pair of fins. In a 

 single genus it is contained in a light and symmetrical monothalamous 

 shell. Compared vith the Nautilus, the cephalic tentacula are much 

 reduced in number, the external ones, continued from the oral sheath, 

 not exceeding eight, to which, in most of the genera, is added a pair 

 of internal and much longer tentacula. The arms are much increased 

 in size and of a more complicated structure, supporting on their in- 

 ternal surface numerous suckers, and sometimes connected together 

 by powerful muscular web. The eyes are much larger and more 

 complex, are no longer pedunculated, but lodged in orbits. The 

 mouth is armed with two piercing and trenchant horny jaws, resem- 

 bling in shape and in their vertical movements those of the Nautilus. 

 The gills are two in number, each with a ventricle expressly appro- 

 priated to the branchial circulation ; the systemic circulation having 

 a single muscular ventricle, as in the Nautilus. The infundibulum is 

 a complete muscular tube, shaped like an inverted funnel. They possess 

 a gland and membranous receptacle for secreting and expelling 

 an inky fluid. The sexual organs are in distinct individuals, as 

 in the Tetrabranchiate order. All the species of both orders of 

 Cephalopods are aquatic and marine. 



The Dibranchiate order may be subdivided into two tribes ; the 

 one provided with the eight ordinary arms and the two longer ten- 

 tacles, hence called Decapoda ; the other tribe without the tentacles, 

 and called Octoj)oda. 



The various forms of the extinct BelemiiitidcB constituted one 

 family in the Decapod tribe. The little Spirula, characterised by a 

 less complex, but internal chambered shell, is the type of a second 

 family. The cuttle-fish, characterised by its internal calcareous shell, 

 which feebly represents that of the Belemnite, exemplifies a third 

 family of Decapods called Sepiadce. The common calamary {Loliyo), 

 in which the internal shell is reduced to a horny quill-shaped plate, 

 repi-esents the fourth and most extensive family of the present tribe, 

 which I have called TeuthidcB ; and in which one genus (^Onycho- 

 teuthis) had the caruncle of more or fewer of its acetabula produced 

 into horny claws. In all the Decapods the mantle supports a pair 

 of fins, and the siphon is generally provided with a valve. 



In the tribe Octojwda, fins are rarely developed from the mantle ; but 

 the eight ordinary arms are longer, thicker, and are united together 

 by a broader web, which forms a powerful organ for swimming in a re- 

 trograde direction. One family in this tribe ( Testaceci) is represented by 

 the genus Argonauta, in which, at least in the female sex, the first or 

 dorsal pair of arms is dilated at its extremity into a broad thin mem- 



