438 



CEPHALOPODA- 



living animal dwells in deep water, and when it comes to the 

 surface is so vigilant against surprise, that at the slightest alarm it 

 sinks to the bottom. On making a section of the shell its cavity 

 is found to be partitioned off by numerous shelly septa into various 

 chambers (Jig. 205, s, s), in the last of which the body of the 

 animal is situated. A long tube, or siphuncle (A, A), partly cal- 

 careous and partly membraneous, passes through all the compart- 

 ments quite to the end of the series. The membranous siphuncle 

 is continued into the animal, and terminates in a cavity contained 

 within its body, hereafter to be described, which is in free com- 

 munication with the exterior. 



Various conjectures have been indulged in concerning the 

 end answered by the camerated condition of the shell in these 

 MOLLUSCA. Dr. Hooke* suggested the idea that the chambers 

 might be filled with air generated by the Nautilus, and thus made 

 so buoyant that the specific gravity of the animal and its shell 

 should correspond with that of the surrounding medium, and 

 that, acting in the same manner as the swimming-bladder of a 

 fish, the creature would float or sink, as the air in its shell was 

 alternately compressed and rarefied. Should this supposition be 

 correct, it would seem probable, as Dr. Buckland has pointed out, 

 that the simple retraction of the head, by injecting water from 

 the chamber within its body (pericardium) into the membranous 

 siphuncle, would cause the needful condensation of the air con- 

 tained in this singular float, and allow the Nautilus to sink to 

 the bottom ; while the protrusion of its arms, by taking off the 

 pressure, and thus allowing of the expansion of the confined air, 

 would give every needful degree of buoyancy, even sufficient to 

 permit the mollusk to rise like a balloon to the top of the sea. 



The body of this Cephalopod is covered with a thin mantle 

 (a, a), of which a large fold (b) is reflected on the exterior of the 

 shell. It is securely fixed to its residence by two lateral muscles, 

 the insertion of one of which is seen at g. A large coriaceous 

 hood (n) covers the head, and, when the creature retreats into its 

 habitation, closes the entrance like a door ; while through the infun- 

 dibulum (i) the ova and excrementitious matters are expelled from 

 the body. The most remarkable feature, however, exhibited in 

 the external conformation of Nautilus, is the conversion of the 

 sucker-bearing arms of other Cephalopods into an elaborate appara- 

 tus of tentacular organs appended to the head (o, o) ; but these, as 

 well as the eye (wi), will be more minutely described as we proceed. 



* Philosophical Experiments and Observations, 8vo. 1726. 



