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euclosed witliiii the lower part of the mantle ; — the Nautilus has no arms, 

 but a mass of some thirty or forty sheathed tentacles, a hood or covering, 

 an aditional pair of gills, and a chambered shell with the whorls involuted 

 one upon the other, containing the animal in its outer porch. I proceed at 

 once, therefore, to the consideration of them as genera. 



Aegonauta. Spirula. Nautilus. 



Genus 1. ARGONAUTA, Linnceus. 



Animal; hody siibghhose, croio7ied with eight long arms, each 

 having two roivs of suckers, the two frofit arms furnished with 

 minute secretive vessels and developed at the extremity into an 

 elastic membranous web. Two branchia. 



Shell ; very thin, elastic and permeable to light, boat-shaped, not 

 chambered, slightly convoluted into a discoid spire, ichich is 

 double-heeled and partially immersed within the aperture; keels 

 more or less tuberculated. 



The Paper Nautilus and the Pearly Nautilus Shells, though both of 

 cephalopodic origin, are of very different composition and design. The 

 Pearly Nautilus is a shell of rather elaborate structure, presenting a formi- 

 dable protection to the soft parts, partitioned off into chambers by the aid 

 of which a vacuum is produced of sufficient buoyancy to sustain the great 

 pressure of water to which the animal is exposed in its deep region of habi- 

 tation. The Paper Nautilus, or Argonaut, is merely a light elastic case, 

 constructed by the female of a naked Cephalopod, for the preservation of 

 her eggs; a sexaal provision subservient to the generative economy. No 

 Argonaut shell has been discovered with a male inhabitant, although many 

 have been taken in the Mediterranean with the female, in company Avith 

 the well-known octopod of the Neapolitan market. 



Assimiing the Argonaut to be really the mate of the Octopus, I scarcely 

 know a more beautiful adaptation of means to an end than is to be found in 

 this liideous-looking animal. The two front arms with its suckers are fur- 

 nished mth minute secretive glands and each extremity is modified into an 

 expansile membranous web, endowed with a feeble power of calcification, 

 similar to that in the mantle of other mollusks. They are capable of very 

 considerable expansion, and deposit a thin wavy layer of shell matter form- 

 ing a symmetrical boat or car, the discoid portion of which serves as a 

 receptacle for the eggs. According to the observations of M. Eang, who 

 relates the circumstance of liis having seen several living Argonauts wliilst 

 rowing iji a boat in the Port of Algiers, the vclamentous arms are extended 



