CEPHALOPODOTJS MOLLTJSCA. 249 



Respiration is performed by symmetrical branchiae lodged 

 in a fold of the mantle : in the first order, there are two ; in the 

 second, four of these, organs. There is a fleshy ventricle for 

 the circulation of the blood through the body, and like organs 

 to aid its passage through the gills. The funnel is a mem- 

 branous tube, situated at the under side of the neck ; it gives 

 passage to the water for respiration, and exit to the effete 

 matters that are discharged from the body. The class is 

 divided into two orders : 



The TENTACULIFERA, D'Orb., (TetrabrancUata, Owen,) 

 of which the Nautilus Pompilius is the type, have large 

 external univalve shells, symmetrical in form, and divided 

 internally by septse into a series of chambers, the last 

 formed being very capacious, for lodging the body of the 

 animal. A tube, or siphon, passes through all the chambers, 

 without communicating with them, and opens into a muscular 

 sac that surrounds the heart. 



This apparatus appears to be destined to facilitate the 

 ascent and descent of the animal in the water, by determin- 

 ing an increase or diminution in the specific gravity of 

 the shell, as the pericardial sac communicates with the 

 sea by two openings ; the reservoir and siphon can be 

 distended with water, thereby augmenting the specific 

 gravity of the shell, and causing it to sink ; or emptied by 

 the contraction of its muscular walls, thus enabling it to 

 float. 



The weight of sea- water, which the siphuncular apparatus 

 contains, is the ballast by which the tentaculifera ascend or 

 descend in the water.* The body is attached to the last 

 chamber of the shell by two lateral muscles ; the head is 

 surrounded by numerous hollow tentacula, and they have a 

 muscular disc for creeping. Their eyes are more simple in 

 structure than in the acetabulifera, and are pedunculated. 



They have no organ of hearing ; the jaws are strengthened 

 by a calcareous coating ; they have four gills, but no ink-bag. 

 As the shells of all the tentaculifera (the pearly nautilus 

 excepted) are extinct, the study of these organisms is of great 

 importance to the geologist. We divide this order into three 



* Dr. Wright, Philosophical Magazine, vol. xii. p. 503. 



