610 LECTUllE XXIV. 



pensation. These latter organs are not continued from the muscular 

 cone which corresponds with the cephalic sheath in the Nautilus, but 

 arise, like the internal labial processes in that Cephalopod, close to- 

 gether from the cephalic cartilage, internal to the origins of the 

 ventral pair of arms. They proceed at first outwards to a large 

 membranous cavity situated anterior to the eyes, and emerge between 

 the third and fourth arms on either side. 



In most Octopods the two dorsal arms are the longest : they are ten 

 times the length of the body in Oct. Aranea. But besides their su- 

 perior length, the dorsal arms present other peculiarities in this 

 family of Cephalopods. In the males of Tremoctopus and Argonauta, 

 one of them is enlarged and excavated to receive part of the male 

 sexual apparatus*: in the iem^Xo, Argonauta they are provided, as 

 before stated, with the expanded calcifying membranes (t?^/. 220, c. 1), 

 which are usually spread over the exterior of the delicate shell (a), 

 meeting and overlapping each other along its slender keel. The fabled 

 office of these membranes, as sails to waft the argonaut along the 

 surface of the ocean, and that of the attenuated arms as oars extend- 

 ing over the sides of the boat, have affijrded subject for poetic 

 imagery and constructive analogy in all ages : and the little hypo- 

 thetical navigator of nature's ship has been the subject of the disquisi- 

 tion of the naturalist from Aristotle to Cuvier, and of the song of the 

 poet from Callimachus to Byron. 



In Octopus velifer both the first and second pairs of arms support 

 broad and thin membranous appendages at their extremities. In the 

 common Poulp, Octopus vulgaris, the eight arras are connected to- 

 gether for some distance beyond the head by membranes and muscles, 

 which form a circular fin ; this constitutes its sole locomotive organ 

 when swimming, and, by its powerful contraction, aided, however, by 

 the ejection of the currents from tlie funnel, the animal is propelled 

 through tlie water by a quick retrograde motion. In Oct. semi- 

 palmatus the fin is extended along the basal interspaces of only the 

 fijur dorsal arms. In Sciadephorus it extends between all the arms, 

 and as far as their attenuated extremities. There are two layers of 

 transverse fibres in this web, the external of which arises from a 

 white line along the back part of the base of each arm, the internal from 

 the sides of the same arms between the attachments of the suckers. 

 They decussate one another as they pass from arm to arm in the middle 

 of the webs, and are included between two thin layers of radiating or 

 longitudinal fibres.f 



The internal surface of the arms is that which is specially modified 



* CCCCIX. & CCCCX. t Eschricht, CCCXCIX. p. 627. 



