TETRABRANCHIATA. 443 



Order 1. TETRABRANCHIATA.* 



Cephalopoda with four gills in the mantle-cavity. 



The appendages of the head are peculiar, consisting of a number 

 of lobes carrying sheathed tentacles ; they are described on p. 421. 



The cephalic cartilage, instead of forming a complete ring, consists 

 of two horse-shoe-shaped limbs, on which the central parts of the 

 nervous system lie. The eyes are without a lens or other refractive 

 media. The funnel is formed of two lobes which are not fused. 

 There is no ink sac. The ctenidia are four in number, as also are 

 the branchial vessels, auricles, and kidneys, and probably the 

 osphradia. The pericardium has a pair of external openings, and 

 does not communicate with the kidneys. There is a multilocular 

 external shell, in the last chamber of which the animal lies. It is 

 secreted by the mantle and covers the visceral sac. Its chambers 

 are filled with air and are traversed by a siphon. The shell consists 

 of an external, frequently coloured, calcareous, porcellanous layer, 

 and an internal mother-of -pearl layer. 



The position and structure of the siphon, as well as the form of 

 the septa, and the lines of fusion of the latter with the shell, afford 

 important characters for the classification of the fossil Tetrabran- 

 chiates. 



There is only one living genus confined to the Indian and Pacific 

 Oceans, but the extinct members of the order are numerous and 

 important. 



The siphon (or siphuncle) in Nautilus may have a complete but thin nacreous 

 investment, or only a partial one next one face of the septa, in which case it is 

 called a septal neck (Fig. 338, Sf). In most Nautilidae the septal necks project 

 from the sides of the septa which look towards the apex of the shell, while in 

 the Ammonitidae they are, as a rule, on the other sides of the septa. The 

 siphuncle varies in position ; it may perforate the centre of the septa, in which 

 case it is central or sub-central (Nautilidae}, or it may be marginal, on the 

 outer side of the septa (Ammonitidae). The suture is the name given to the 

 line of union of the edge of the septum with the shell- wall. In the Nautilidae 

 the sutures are uniformly curved or straight ; in Ammonitidae they are exceed- 

 ingly complex, being folded into a number of lobes with the concavity towards 

 the aperture ; the parts of the suture between the lobes are called the saddles, 



* R. Owen, " Memoir on the Pearly Nautilus" London, 1832. J. Van der 

 Hoeven, "Bydraagen tot de ontleedkundige Kennis aangaande Nautilus 

 pompilius," Verhandel. k. Akad. Amsterdam, diel 3, 1856. T. H. Huxley, 

 ''On some points in the Anatomy of Nautilus pompilius," Journ. Proc. 

 Linn. Soc., London, 3, 1859. Keferstein, "Beitr. z. Anat. d. Nautilus pomp.," 

 Nachrichtsbl. k. Ges, wiss. Qottingen, 1865. Lankester and Bourne, "On the 

 existence of Spengel's olfactory organ and of paired genital ducts in the Pearly 

 Nautilus," Q. J. M. S., vol. 23, 1883. J. Graham Kerr, "On some points in the 

 Anatomy of Nautilus pompilius,"' Proc. Zool. Soc. London, 1895. A. AVilley, 

 Q. J. M. S., 39, 40. 



