THE FOUR-GILLED GROUP 



3307 



To the same group as the Spirula belongs the extinct family of 



Belemnitidce, ranging from the L,ias to the Chalk, and whose skeletons 



are commonly known as thunderbolts. They possess a tapering chambered shell, 



inserted into the summit of a long spear-like guard. Most of the species belong to 



the typical genus Belemnites. 



FOUR-GILLED GROUP Order TETRABRANCHIATA 

 Family NAUTlLIDsE 



The nautilus is the sole living representative of this order, and although not so 

 rare as the spirula, the animal of the nautilus is by no means common in collections. 

 It is probably an inhabitant of deepish water, and only likely to be obtained alive 

 by dredging, although a few specimens have been occasionally captured at the sur- 

 face. The animal is contained within the last compartment (A] of a chambered 

 shell, within which it is completely retractile. It does not resemble any of the 

 dibranchiate cephalopods, having numerous small retractile feelers or tentacles, 

 without any suckers, in place of the eight or ten sucker-bearing arms of that order. 

 The beaks are very solid and calcareous, not entirely horny as in the dibranchiates. 

 The eyes are small, and raised on short stalks; the funnel is not a complete tube, 

 being formed of two lobes which fold over one another, but are not joined. To 

 the posterior end of the body is attached a slender fleshy cord, termed the siphuncle 

 (a), which passes through holes in the septa of the shell up the coiled spire. It 

 is inclosed in a horny tube, which is again coated with a calcareous deposit. The 

 function of the siphuncle probably is to preserve the vitality of the first formed 

 portion of the shell, which without some such means of preservation would be 

 liable to decay. The animal is somewhat feebly attached to the shell by two large 

 adductor muscles, one on each side of the body, which are, as it were, connected by 

 a muscular girdle of the mantle passing round the body from muscle to muscle. 

 The chambered shell is beautifully pearly within, but has an external porcelaneous 

 coating. A full-grown shell has about thirty-six septa, which are relatively 

 equidistant, showing that the growth of the animal is regular and gradual 

 throughout life. The septa give immense strength to the shell, sufficient to 

 resist the pressure of the water at great 

 depths upon the air chambers between them. 

 These air chambers undoubtedly serve to 

 buoy up the shell when the animal is swim- 

 ming or desires to rise to the surface; but 

 the old stories of its filling the cells at 

 pleasure with either air or water, and so 

 rising to the surface or descending to the 

 bottom, are mere fables, and comparable to 

 the legends respecting the sailing of the 

 argonaut. The shells of the male and 



female are said to present certain slight SECTION OF SHEW, OF PEARI.Y NAUTILUS. 

 differences. Very little is known of the (Much reduced.) 



