482 PHYLUM MOLLUSCA 



remarkable are large twisted shells, called Hippuntes (Rudistae), whose 

 remains are often very abundant in deposits of the chalk period. 



Class V. Cephalopoda. Cuttlefish 

 Examples. — Sepia, Octopus (Polypus), Loligo, Nautilus 



The Cephalopods are bilaterally symmetrical and free- 

 swimming. The head is surrounded by numerous *' arms " 

 bearing tentacles or suckers. These arms seem to be equivalent 

 to processes of the margin of the foot. Another portion of the 

 foot forms a partial or complete tube— the " siphon " or 

 ''funnel " — through which water is forcibly expelled from the 

 mantle cavity. The muscular mantle flap which shelters 

 well-developed plumose gills is posterior in position ; the 

 visceral hump shows no trace of spiral coiling, but is elongated 

 in a direction anatomically dorsal and posterior, though it 

 may point forwards when the animal propels itself through 

 the water. Except in the Pearly Nautilus, the shell of 

 modern forms has been enclosed by the mantle, and is, in most 

 cases, only hinted at. There is a very distinct head region, 

 furnished with eyes and other sensory structures, and the 

 mouth has strong beak-like jaws, as also a well-developed 

 radula. The nervous system shows considerable specialisa- 

 tion ; the chief ganglia are concentrated in the head, and 

 sheltered by cartilage. The true body cavity, pericardium of 

 other Molluscs, is usually well developed, and frequently 

 surrounds the chief organs. Except in the Nautilus, it com- 

 municates with the exterior by the nephridia. The nephridia 

 are disposed on the walls of the afferent branchials. 



The vascular system is well developed, and, except in the 

 Nautilus, there are accessory branchial hearts. The sexes are 

 separate. The gonad is in a coelomic sac and not directly 

 continuous with the gonoduct. The ovum undergoes incom- 

 plete segmentation. Development is direct. In habit, 

 Cephalopods are predominantly active and predatory ; in 

 diet, carnivorous. 



The shells of the Nautilus are common on the shores 

 of Far East seas, but the animals are less familiar. The 

 Nautilus swims quickly after crustaceans along the bottom at 

 30-70 fathoms, and its appearance on the surface, *' floating 

 like a tortoiseshell cat," is probably the result of injury. 



