MOLLUSCA. /----------------------—|---------------------------\ _Classes._ Cephalopoda. Pteropoda. Lamellibranchiata. | Gasteropoda. Brachiopoda. | /----|---------------------------------------\ _Orders._ Dibranchiata. Tetrabranchiata. | | /----|-------------—\ /-------------|------------\ _Sections._ Decapoda. Octopoda. Nautilidæ. Ammonitidæ. | | Orthoceratidæ. | +------------------------+ | | /----|-----------------------\ /--------—|---------—\ _Families._ Teuthidæ. Sepiidæ. Spirulidæ. Octopidæ. Argonautidæ. | Belemnitidæ. | | | | | | | /--|---—\ /-----—|--—\ /----|-----\ | _Genera._ The Squids: The Cuttles: The Octopus; | _Loligo_, &c. _Sepia_, &c. _O. vulgaris_, &c. | | /-----------|-----\ The Paper Nautilus: _Argonauta_. It will be seen that it may be said to be first cousin to the Argonaut, or “Paper Nautilus,” and second cousin to the cuttle and squid. The Argonaut branch of the family is in possession of all the house property, which seems to have been entailed on the female line; for the paper-nautilus is, in fact, a female octopod provided with a shell in which to carry and protect her eggs. Instead of the whole of the eight arms tapering to a point, as in the octopus, two of the dorsal limbs are flattened out at their extremity, and from their membranes she secretes, and, if necessary, repairs the shell, and, by applying them closely to its outer surface on each side, holds herself within it; for it is not fastened to her body by any attaching muscles.[6] The male argonaut is very small,—not more than an inch in length,—and has no shell. Hence, even by eminent naturalists, as Dumeril and De Blainville, it was long regarded as doubtful whether the shell was really secreted by the female, or whether, like the hermit-crab, she borrowed for her protection the empty habitation of some other mollusc.