830 
CLASS XIII. 
Rossia OWEN. 
Loligopsis Lam., Perothis KscuscHouTz, RATHKE. 
LAmarckK has described this genus as having only eight arms; the long 
tentacula are often torn off. In common with Cranchia this genus has no 
membranous valve in the funnel. These animals are nearly transparent; 
the eight arms are short; the body is conical, posteriorly narrow and with 
two round fins at the extremity. Compare RaTHKE Perothis ein neues 
Genus der Cephalopoden, Mém. presentés & V Acad. impér. des Sc. de St Peters- 
bourg, 1. 1835, pp. 149—176. 
Cranchia LEACH. 
Sp. Cranchia scabra LEAcH in TucKEY’s Exped. to the River Zaire, p. 410. 
The fins are situated quite at the end of the body and the dorsal surface of 
the mantle is grown fast to the head. f 
Family XIV. Octocera s. Octopoda. Arms eight, large, often 
very long, surrounding the mouth in a circular row. Body sacci- 
form, without fins. Funnel without valve. 
Argonauta L. Arms furnished with a double row of acetabula 
(suckers), the two superior expanded into a membrane towards the 
extremity. Shell thin, involute, external, unilocular, with spire 
bicarinate. 
This remarkable shelled animal excited long ago the admiration of the 
ancients ; see PLINIUS ‘Hist. Nat. 1x. cap. 29. It was supposed that it made 
use of its fin-shaped arms as a sail, and thus in still weather could swim on 
the surface of the sea; but the observations of Rana (GuséRIN Magazin 
de Zool. 18371) have not confirmed this opinion; these arms lie expanded 
along the outside of the shell and serve to fasten the animal in its shell, 
which is not attached to it by any muscles. Many naturalists (RAFINESQUE, 
LeaAc# and others) thought, that this animal like the hermit-crab (Pagurus) 
lived parasitically in a borrowed shell, and that the shell of Argonauta 
belonged to some unknown species of mollusc. Accordingly this genus of 
Octopoda was named as a new genus Ocythoé. Although this animal can 
readily move from its shell, just like some Pteropoda, this opinion is now, 
however, sufficiently refuted. For the discrepant form of the male indivi- 
duals which have no shell see above, p. 820—823. 
On the anatomy of Argonauta compare Poxt Test. utr. Sicil. Tom. 111. 
and VAN BENEDEN Now. Mém. del’ Académie royale des Sc. de Bruselles, 
Tom. xt. 1833. 
1 Pow! adopts this notion of the ancients, and he has even figured the animal in 
accordance with it (Testac. utr. Sicil. 111. Tab. 40), but says that he has never himself 
seen the sails of the Argonauta. 
