494 Miss Agnes Crane — Recent and Fossil Cep/ialopoda. 



almost as suddenly as some of the more ancient members of tliat 

 tetrabranchiate group, of which they were nearly the last representa- 

 tives. 



In the more highly organized, but, geologically speaking, modern 

 class of dibrancMate, or two-gilled Cephalopods, the shell is internal, 

 and rudimentary in the Octopoda, either horny as in the squids, or 

 calcareous and laminated as in the cuttles. In these ''naked" forms, 

 respiration is effected by means of two gills instead of four. The 

 dibranchs are active free-swimming animals possessing super-added 

 branchial hearts, and, therefore, a more complex and vigorous 

 circulatory sj'stem, which gives them increased powers of locomotion. 

 The ink-bag is always present more or less developed, and its 

 preservation in a fossil state is regarded by Professor Owen as certain 

 evidence of the existence of forms unprotected by an external 

 defensive shell. The parrot-like mandibles are horny, and are quite 

 hidden from view at the base of the web uniting the arms to the 

 body. In the Octopoda, or eight-armed species, it is important to 

 remember, they form the only portions of the animal susceptible of 

 preservation in sedimentary strata. In the cuttles, the two long 

 additional tentacular arms are retractible, and are only extended 

 when the animal is capturing its living prey, or in the act of dissolu- 

 tion. In some species of the Calamaries or squids, they are rendered 

 still more formidable weapons of offence and defence by the addition 

 of a double series of horny hooks attached to the club-like ends of 

 the tentacles. In the anomalous genus Argonaut, or Paper Nautilus, 

 the Nautilus primus of Aristotle, the position of the one-celled shell 

 is externaU It is, however, believed to be peculiar to the female, 

 and, not being attached to the animal by shell-muscles, is considered 

 as serving more probably for the protection and incubation of the 

 eggs. Two species of this genus occur, fossil, in later Tertiary 

 deposits. In Spirula, again, the shell, though internal, is many- 

 chambered and siphunculated. Dead shells of this genus abound in 

 the warmer seas, but the animal has been but rarely obtained, 

 except in a more or less fragmentary condition. Specimens were 

 fortunately procured during the cruise of the " Challenger," and will 

 receive a detailed description from Professor Huxley. Sjpirula is an 

 exceptional form that may possibly be found to possess characters 

 uniting the dibranchiates to their more lowly organized predecessoi's, 

 and will, at any rate, throw additional light on the structure of the 

 fossil members of the order to which, from the presence of an ink- 

 bag, it was originally referred by Professor Owen.^ 



Of the existence of this highly-organized class of Cephalopods 

 prior to the Secondary period, we have purely negative evidence. 

 If any of their forms inhabited the Palgeozoic seas, they have left no 

 certain traces for the instruction of the naturalist. The earliest 



1 This must be understood in a modified sense. The fact is the two broadly- 

 expanded (shell-secreting?) dorsal arms (figured as "sails" in the allegorical 

 representations of the "Argonaut") closely envelope the shell on either side, so that 

 in the lifetime of the animal the shell is almost, if not wholly, concealed ; whereas 

 the animal can but partially conceal itself within the shell. — H. W. 



« Yoyage of H.M.S. Samarang (Zoology), pp. 6-17, pi. 4. 4to. 1848. 



