342 



MOLLUSCS. 



allied genera. This list is usually swelled by the addition 

 of the extinct Ammonites, but there are some reasons for 

 believing that these belong to the cuttlefish section of 

 Cephalopods. 



Cephalopoda, 



Tetrabranchiata {Nautilus), 



All extinct except one genus — 

 Nautilus ; the extinct forms 

 are usually ranked as Nauti- 

 loid 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 num- 

 ber of lobes, which carry ten- 

 tacles in little sheaths, and no 

 suckers. 



The two mid-lobes of the foot 

 form a siphon, but they are 

 not fused into a tube. 



The eye is an open sac without a 

 lens. There are two " os- 

 phradial papillae " or smelling 

 patches at the bases of the 

 gills. 



Two pairs of gills ; two pairs of 

 nephridial sacs ; two genital 

 ducts (the left rudimentary). 



Theviscero-pericardial sacopens to 

 the exterior by two apertures. 



The heart has no auricles, and 

 there are no branchial hearts. 



No ink-bag. 



DiBRANCHiATA {Sepia, Octopus, etc.). 



Numerous living genera, ranked 

 as Decapods or Octopods ; 

 along with the former the 

 extinct Belemnites are in- 

 cluded. 



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 degenera- 

 tion (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 

 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 

 nephridial sacs ; two oviducts 

 in Octopoda and Ommas- 

 trephes ; two vasa deferentia in 

 Eledone moschata ; in others 

 an unpaired genital duct. 



The viscero-pericardial sac opens 

 into the nephridia by two pores. 



The heart has two auricles, and 

 there are branchial hearts. 



An ink-bag. 



