428 PHYLUM MOLLUSCA. 
Class V. CepHALopopa. 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 ts 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 ts, in most 
cases, only hinted at. There is a very distinct head region, 
jurnished with eyes and other sensitive structures, and the 
mouth has strong beak-like jaws, as also a well-developed 
radula, The nervous system shows. considerable specialisa- 
tion; the chief gangla are concentrated in the head, and 
sheltered by cartilage. The true body cavity, pericardium of 
other Molluscs, ts 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 ts in a celomic sac and not directly 
continuous with the gonoduct. The ovum undergoes incom- 
plete segmentation. Development is direct. ln habit, 
Cephalopods are predominantly active and predatory ; in diet, 
carnivorous. 
The shells of the pearly Nautilus are common on the 
shores of warm seas, but the animals are much less familiar. 
The Nautilus creeps or swims gently along the bottom at 
no great depth, and its appearance on the surface, “ floating 
like a tortoiseshell cat,” is probably the result of storms. 
It is called “pearly” on account of the appearance of the 
