4 I2 



PHYLUM MOLLUSC A. 



allied genera. This list is usually swelled by the addition of 

 the extinct Ammonites, but there are some reasons for 

 suspecting that these belong to the Dibranchiate section of 

 Cephalopods. 



The following table states the chief points of distinction 

 between Nautilus and the other series of Cephalopods ;- 



CEPHALOPODA. 



TKTRABRAXCHIATA (Nautilus). 



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, 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 smelling 

 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 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. 



DIBRANCHIATA (Sepia, Octopus, etc.). 



Numerous living genera, ranked as 

 Decapods or Octppods ; along with 

 the former the extinct Ilelemnites 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 

 Oinmastrephes', two vasa deferentia 

 in Eledone luoschata ; in ethers an 

 unpaired genital duct. 



The coslom 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 (see Table). 



Family I. Nautilidre. Nautilus alone alive ; but a great 

 series of fossil forms, Orthoceras Trochoceras. 



Family II. Ammonitidie. All extinct, but with shells well 

 preserved, so that long series can be studied. They 

 furnish striking evidence of progressive evolution in 



