CLASSIFICATION OF CEPHALOPODS 



485 



suspecting that these are rather " convergent " than 

 nearly related. 



The following table states the chief points of distinction 

 between Nautilus and the other series of Cephalopods : — 



Cephalopoda 



Tetrabranchiata (Nautilus). 



All extinct except one genus — Nautilus ; 

 • the extinct forms are usually ranked 

 as Nautiloid and Ammonoid. 



Shell externa], chambered, straight or 

 bent or spirally coiled. That in which 

 Nautilus lives has been described, 

 with its siphuncle, gas-containing 

 compartments, etc. 



The part of the foot surrounding the 

 mouth bears a large number of lobes, 

 which carry tentacles in little sheaths, 

 but no suckers. 



The two mid-lobes of the foot form a 

 siphon, but they are not fused into a 

 tube. 



The eye is without a lens, and is bathed 

 internally by sea-water, which enters 

 by a small pinhole aperture. There 

 are two " osphradia " or smelUng 

 patches at the bases of the gills. 



Two pairs of gills ; two pairs of ne- 

 phridia ; two genital ducts (the left 

 rudimentary). 



The coelom sac (pericardium) opens 

 directly to the exterior by two aper- 

 tures. 



The heart has two pairs of auricles, and 

 there are no branchial hearts. 



No ink-bag. No salivary glands. 



DiBRANCHiATA (Septa, Octopus, etc.). 



Numerous living genera, ranked as 

 Decapods or Octopods ; along with 

 the former the extinct Belemnites are 

 included. 



No living Dibranchiate lives in a shell. 

 The shell was internal even in the 

 extinct Belemnites, and in modern 

 forms it occurs in various degrees of 

 degeneration (cf. Spirula, Sepia, 

 Loligo), or is quite absent (Octopoda). 



The part of the foot surrounding the 

 mouth is divided into ten or eight 

 arms, which carry suckers, stalked in 

 Decapods, sessile in Octopods. 



The two mid-lobes of the foot fuse to 

 form a completely closed tubular 

 siphon or funnel. 



The covering of the eye may be per- 

 forated, but the mouth of the retinal 

 cup is closed by a lens. There are no 

 osphradia, though there may be 

 " olfactory pits " behind the eyes. 



One pair of gills ; one pair of nephridial 

 sacs ; two oviducts in Octopoda and 

 Oigopsida ; two vasa deferentia in 

 Eledone moschata ; in others an un- 

 paired genital duct. 



The ccelom opens into the nephridia 

 by two pores, and thus to the ex- 

 terior. 



The heart has two auricles, and there 

 are branchial hearts. 



Usually with an ink-bag. Salivary 

 glands. 



Classification of Cephalopoda 



Order L Tetrabranchiata (see Table). 



Family I. Nautilida^. Nautilus alone alive ; but a great 

 series of fossil forms, e.g. Ofthoceras, Trochoceras. 



Family XL Ammonitidce. All extinct, but with shells well 

 preserved, so that long series can be studied. They 

 furnish striking evidence of progressive evolution in 

 definite directions, e.g. Bactrites, Ceratites, Baculites, 

 Ttirrilites, Heteroceras, and the whole series of genera 

 formerly classed as Ammonites. 



