N A U 



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NEC 



ingeniously and beautifully explained by 

 Prof. Buckland. The siphuncle, as ap- 

 pears from Mr. Owen's statement, ter- 

 minates in a large sac surrounding the 

 heart of the animal ; if we suppose this 

 sac to contain a pericardial fluid, the 

 place of which is alternately changed from 

 the pericardium to the siphuncle, we shall 

 find in these organs an hydraulic appa- 

 ratus for varying the specific gravity of 

 the shell ; so that it sinks when the peri- 

 cardial fluid is forced into the siphuncle, 

 and becomes buoyant, when the same 

 fluid returns to the pericardium. The 

 substance of the siphuncle is a thin and 

 strong membrane, surrounded by a coat 

 of muscular fibres, by which it could con- 

 tract or expand itself, in the process of 

 admitting or ejecting any fluid to or from 

 its interior. When the arms and body 

 are expanded, the fluid remains in the 

 pericardium, and the siphuncle is empty, 

 and collapsed, and surrounded by the 

 portions of air that are permanently con- 

 fined within each chamber ; in this state 

 the specific gravity of the body and shell 

 together is such as to cause the animal to 

 rise, and be sustained floating at the sur- 

 face. When, on any alarm, the arms and 

 body are contracted, and withdrawn into 

 the shell, the retraction of these parts, 

 causing pressure on the pericardium, 

 forces its fluid contents into the siphun- 

 cle ; and as the quantity of matter within 

 the shell is thus increased, without any 

 increase of magnitude, the specific gra- 

 vity of the entire animal is increased, 

 and it begins to sink. Rumphius states 

 that, at the bottom of the sea, the nautilus 

 creeps with his boat above him. 



Fossil remains of the nautilus are found 

 in strata from the mountain limestone 

 upwards. In some of these the siphun- 

 culus is beautifully preserved. But while, 

 as a genus, the nautilus occurs in forma- 

 tions of every age, from the transition 

 series upwards, yet certain species appear 

 limited to particular geological formations : 

 thus the nautilus multicarinatus is limited 

 to strata of the transition formation ; the 

 nautilus bidorsatus, to the muschelkalk ; 

 the nautilus obesus, and nautilus lineatus, 

 to the oolite ; the nautilus elegans, and 

 nautilus undulatus, to the chalk forma- 

 tion. The eocene, miocene, and pliocene 

 has each its particular nautili. Buck- 

 land. Kir by. Owen. Parkinson. 

 Sowerby . 



NAU'TILUS SY'PHO. The name given to a 

 very beautiful, camerated, siphuncled fos- 

 sil shell found in the tertiary strata at 

 Dax, near Bourdeaux. This fossil pre- 

 sents deviations from the usual characters 

 of the nautilus, whereby it approximates 

 to the ammonite. 



NAU'TILUS zic ZAC. A fossil, camerated, 

 siphuncled shell, found in the London 

 clay. This and the nautilus sypho ap.- 

 pear to form connecting links between 

 the genera Nautilus and Ammonite. 



NE'BULA. (nebula, Lat.) A cloud ; a 

 cluster of stars. It is to Sir Wm. Her- 

 schel that we owe the most complete 

 analysis of the great variety of those 

 objects which are generally classed under 

 the common head of Nebulae, but which 

 have been separated by him into, 1st, 

 clusters of stars, in which the stars are 

 clearly distinguishable ; and these, again, 

 into globular and irregular clusters ; 2nd , 

 resolvable nebula, or such as excite a 

 suspicion that they consist of stars, and 

 which any increase of the optical power 

 of the telescope may be expected to re- 

 solve into distinct stars ; 3d, nebulae, 

 properly so called, in which there is no 

 appearance whatever of stars ; which, 

 again, have been subdivided into subor- 

 dinate classes, according to their bright- 

 ness and size ; 4th, planetary nebulae ; 5th, 

 stellar nebulae ; and, 6th, nebulous stars. 

 Not fewer than 2000 nebulae, and clus- 

 ters of stars, were observed by Sir Wm. 

 Herschel, whose places have been com- 

 puted from his observations, reduced to a 

 common epoch, and arranged into a cata- 

 logue, in order of right ascension. In a 

 paper read before the Royal Society, Sir 

 John Herschel gives the places of 2,500 

 nebulae and clusters of stars ; of these 500 

 are new. Nebulae have great variety of 

 forms. The distribution of the nebulae 

 over the heavens is even more irregular 

 than that of the stars. In some places 

 they are so crowded together as scarcely 

 to allow one to pass through the field of 

 the telescope before another appears, while 

 in other parts, hours elapse without a 

 single nebula appearing. Sir J. Her- 

 schel. Mrs. Somerville. 



NE'BULAR. Pertaining to nebulae. 



NE'BULOUS. Cloudy ; hazy. 



NEBULO'SITY. The state of being cloudy. 



NECRO'PHAGOUS. (from vtKpbg, dead, and 

 0ayau, to eat, Gr.) Animals which de- 

 vour dead substances. The unclean 

 animals, with respect to their habits and 

 food, were divided into two classes ; 

 namely, zoophagous animals, or those 

 which attack ami devour living animals ; 

 and necrophagous animals, or those which 

 devour dead ones, or any other putres- 

 cent substances. 



NE'CROMITE. (from veicpog, dead, Gr.) 

 A mineral occurring in small masses in 

 limestone ; found near Baltimore. When 

 struck it exhales a fetid odour, resem- 

 bling that of putrid flesh ; from this 

 quality it obtained its name. 



NE'CTARY. That part of a flower which 



