HEAD-FOOTED MOLLUSCS 311 



which is generally considered a kind of organ of smell, entrusted 

 with the duty of testing the quality of the water which enters 

 the pallial cavity. It is termed the osphradium. 



The preceding account of the Ormer illustrates the most 

 prominent characters of Mollusca generally, which are: (i) the 

 absence of segmentation, (2) the presence of a mantle, (3) the 

 muscular foot, (4) the systemic heart, (5) plume-like gills, and 

 (6) a nerve-ring surrounding the first part of the digestive tube. 

 The vast majority of Molluscs either possess all these characters 

 or else a sufficient number of them to leave no doubt as to 

 how they should be classified. Other very common, though by 

 no means universal, characters of the group are the presence 

 of a shell and development of a rasping organ (odontophore). 

 Large numbers of Molluscs are also distinguished by the 

 bilateral symmetry of their bodies, and though the Ormer is 

 not one of these, it is, as already pointed out, symmetrical so 

 far as the lower half of the body is concerned. 



Five classes are recognized among Mollusca, as follows: — 



1. Head -footed Molluscs (Cephalopoda), including the Pearly 

 Nautilus, Cuttle-Fishes, Squids, and Octopi. 



2. Snails and Slugs (Gastropoda). 



3. Bivalve Molluscs (Lamellibranchia), including forms with the 

 shell in two pieces, e.g. Oyster, Mussel, and Cockle. 



4. Tusk-shells (Scaphopoda). 



5. Proto-Molluscs (Amphineura), a small group of which the only 

 common member is Chiton, distinguished by the possession of eight over- 

 lapping shelly plates on the upper surface of the body. 



Class i.— HEAD-FOOTED MOLLUSCS (Cephalopoda) 



As a good type for description we may select the Common 

 Cuttle- Fish [Sepia officinalis), one of our native species, which 

 preys upon fishes and Crustacea in shallow water, and is a free- 

 swimming form (fig. 179). 



External Characters. — The body is bilaterally symmetrical, 

 and at one end of it the moiith may be seen, provided with a 

 pair of horny jaws resembling those of a parrot, and surrounded 

 by ten arms or tentacles, of which two are very long and can be 

 drawn back into special pouches. The inner sides of the eight 

 short arms are studded with adhesive suckers, and each long 

 arm swells at its end into an oval pad, one side of which is 



