Clans 4. Cephalopoda. Order 1. TetrabrancMata. 323 



The Tetrabranchiata are at tlie present day only represented by the genus 

 Nautilus, of whioh there are two species in the Indian and the Pacific Oceans ; 

 they may crawl upon the ground or swim at the surface. In former times (even 

 as early as the Silurian) this division was very numerous, partly represented by 

 forms with straight or curved shells (c/., p. 318). 



The Ammonites are a large group of extinct animals, in which the shell 

 is multilocular like that of the Nautilus, with perforated septa, spiral or straight, 

 curved, etc., but differing from it in that the pei-forations lie close to the convex 

 side, whilst, in Nautilus, they are usually median ; and also in that the septa are 

 strongly sinuous at the fusion with the shell : many were possessed of an 

 operculum (aptychus). The Ammonites first occur in the Silurian formations, 

 and die out in the Cretaceous. Their systematic position is quite uncertain; 

 they are mentioned here on account of the resemblance the shell bears to that of 

 Nautilus, but it cannot be affirmed that they are allied to it. 



Order 2. DibrancMata. 



There are eight or ten arms with suckers. The funnel is tubular ; 

 the eyes have a lens. There are two gills (two auricles, two kidneys) ; 

 an ink sac ; an internal shell, or, none. 



1. Decapods (Decapoda). Ten arms, suckers stalked, and with chitinous 

 rings ; shell present, body elongate and with fins. To this group belong, for 

 example : Sepia officinalis, which occurs abundantly in European seas, and whose 

 thick shell, formed of fine calcareous laminae {os sepise of the chemist) is 

 turned to various technical uses. Here too, belongs the Sea-clerk (Loligo 

 vulgaris), occurring in the same places, and possessed of a thinner horny shell. 

 Further, the gigantic Architeuthus, a pelagic animal several metres long, but not 

 otherwise differing from the common decapod type. The so-called "thunder- 

 bolts," which are especially abundant in the Cretaceous strata, and which, on 

 account of the displacements, consequent on the Glacial Period, occur- also in the 

 glacial deposits of North Eui-ope, are the posterior tongue-shaped ends of the 

 shells of certain extinct Oephalopods (Belemnites). 



2. Octopods (Oetopoda). Eight arms, suckei-s sessile and without homy 

 rings, no shell ; thick body without fins. Here belongs, for example, Octopus 

 vulgaris, a large animal with long arms, small round body ; abundant in the 

 MediteiTanean. Also Argonauta argo, the females of which are characterised by 

 the compression of the first pair of arms, and theii- extension posteriorly, to 

 foi-m two lamellae, reaching round the body ; these secrete, on the inner side, a 

 thin, cap-like calcai-eous shell, to protect the body, and to contain the eggs. It 

 is at no point closely adherent to the upper surface of the animal. The male 

 possesses a hectocotylus, but the first pair of arms is noi-mal in sti-ueture, and it 

 has no shell. Argonauta is a pelagic animal. 



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