EXO-SKELETON. ENDO-SKELETON. 567 



markable feature, however, exhibited in the external conformation of 

 Nautilus is the conversion of the sucker-bearing arms of other Cepha- 

 lopoda into an elaborate apparatus of tentacular organs appended to 

 the head (o o) ; but these, as well as the eye (m), will be more minutely 

 described as we proceed. 



(1520.) Turning our attention to the anatomical structure of the 

 CEPHALOPODA, we find that in all of them the exterior of the body is 

 entirely formed by an intricate interlacement of muscular fibres. The 

 sac that contains the viscera, itself muscular, is united to the head by 

 strong and largely- developed fasciculi ; the funnel (fig. 285, a), through 

 which, as through a fleshy pipe, the products of excretion, as well as 

 the eggs or seminal fluid, are ejected, is formed of a tissue similarly 

 endowed with contractility ; while the arms are composed externally of 

 muscles disposed in various directions, and moreover have their central 

 portion occupied by strong bands, which traverse them longitudinally 

 from end to end, so that they are thus gifted with all needful powers 

 of motion, and may be shortened, elongated, or bent in any direction at 

 pleasure. 



(1521.) In those natatory species which, like Loligopsis, or Onycho- 

 teuthis (fig. 282), have fins appended to the sides of the visceral sac, 

 these organs likewise are made up of muscular substance ; and, being 

 thus converted into broad moveable paddles, they also form efficient 

 locomotive agents. 



(1522.) One important circumstance observable in the class before 

 us must not be forgotten in connexion with this portion of the history 

 of the Cephalopoda. We may remind the student that, in the verte- 

 brate division of animated nature, to which these creatures immediately 

 lead us, the locomotive system is supported by an internal vascular and 

 living skeleton, composed either of cartilage, as is the case in the most 

 imperfect vertebrated genera, or, in the more highly organized forms, of 

 bones articulated with each other, and possessing- within themselves 

 the means of growth and renovation derived from the blood which 

 permeates them in every part. The reader will remember that, in all 

 the classes that have offered themselves to our notice, we have not 

 hitherto observed anything at all comparable to an internal osseous 

 framework such as Man possesses, dead, extravascular shells, formed 

 by successive depositions of layers of calcareous material, or jointed 

 cuticular armour equally incapable of growth, having as yet repre- 

 sented the skeleton, and formed the only levers upon which the muscular 

 system could act in producing the movements connected with loco- 

 motion. 



(1523.) Having, however, already had abundant opportunities of 

 seeing how gradually nature proceeds in effecting the development of 

 a new series of organs, we might naturally be led to expect in the 

 creatures before us some faint indications, at least, of our approach to 



