344 LECTURE XXIV. 



in the anomalous genus Cirroteuthis, in which they are attached to 

 the anterior part of the sides of the mantle. 



The sole locomotive organs in the ordinary Octopods, and the sole 

 prehensile organs in all the Dibranchiata are the appendages deve- 

 loped from the head, termed ' arms,' ' feet,' ' tentacles,' and ' pro- 

 boscides.' They have no true homology with the locomotive members 

 of the Vertebrata, but are analogous to them, inasmuch as they relate 

 to the locomotive and prehensile faculties of the animal.* 



The eight arms of the Octopus commence by a hollow cone of 

 muscular fibres attached by a truncated apex to the anterior part of 

 the cephalic cartilage. The fibres are for the most part oblique, and 

 interlace with one another in a close and compact manner, as the cone 

 advances and expands to form the cavity containing the mandibulate 

 mouth, at the anterior extremity of which they are continued forward, 

 and separate into eight distinct portions which form the arms. The de- 

 velopment of the eight external arms bears an inverse proportion to that 

 of the body : they are longest in the short round-bodied Octopi, and 

 shortest in the lengthened Calamaries and Cuttle-fishes, in which the 

 two elongated retractile tentacles are superadded by way of compen- 

 sation. These latter organs are not continued from the muscular 

 cone, which correspond 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 genus Argonauta they are provided, 

 as befoi'e stated, with the expanded calcifying membranes, which are 

 \isually spread over the exterior of the delicate shell, meeting and 

 overlapjiing 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 extending over the 

 sides of the boat, have afforded a beautiful subject for poetic and 

 pictorial imagery and philosophic analogy in all ages : and the little 

 hypothetical navigator of nature's ship has been the subject of the 

 disquisitions of naturalists from Aristotle to Cuvier, and of the song 

 of the poet from Callimachus to Byron. 



* GeofTroy St. Hilaire regards tlio cuttle-fish as a vertebrate animal bent double, 

 with the approximated arms and legs extending forwards : a similar comparison 

 may be found in Aristotle. 



