CLASSIFICATION OF CEPHALOPODA. 



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CEPHALOPODA. 



TETRABRANCHIATA (Nautilus). 



DIBRANCHIATA (Sepia, Octopus, &>c.). 



All extinct except one genus 

 Nautilus ; the extinct forms are 

 usually ranked as Nautiloid and 

 Ammonoid. 



Shell external, chambered, straight 

 or bent or spirally coiled. That in 

 which Nautilus lives has been 

 described, with its siphuncle, gas- 

 containing compartments, &c. 



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 ' ' os- 

 phradia" or smelling patches at 

 the bases of the gills. 



Two pairs of gills ; two pairs of 

 nephridia; two genital ducts (the 

 left rudimentary). 



The coelome sac opens directly to the 

 exterior by two apertures. 



The heart has two pairs of auricles, 

 and there are no branchial hearts. 

 No ink bag. No salivary glands. 



Numerous living genera, ranked as 

 Decapods or Octopods ; along 

 with the former the extinct Bel- 

 emnites are included. 



No living Dibranchiate lives in a 

 shell. The shell is internal even 

 in the extinct Belemnites, and in 

 modern forms it occurs in various 

 degrees of degeneration (cf. Spir- 

 ula, 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 

 perforated, 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 neph- 

 ridial sacs ; two oviducts in Octo- 

 poda and Ommastrephes ; two vasa 

 deferentia in Eledone moschata ; 

 in others an unpaired genital duct. 



The coelome opens into the nephridia 

 by two pores, and thus to the 

 exterior. 



The heart has two auricles, and 

 there are branchial hearts. 



An ink bag and salivary glands. 



CLASSIFICATION OF CEPHALOPODA. 



Order I. Tetrabranchiata (s> 

 Family I. Nautilidae. 



25 



table). 



Nautilus alone alive ; but a great series 

 of fossil forms, Orthoceras Trochoceras. 



Family II. Ammonitidse. 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, 

 Turrilites, Heteroceras, and the whole series of genera 

 formerly classed as Ammonites. 



