Reproduction and Development. 13 



the gill lamellae, while the Paper-Nautilus, Argonaufa, secretes 

 a special shell from the dorsal tentacles, in which the young 

 develop. k 



Special courtship habits are not unknown among Mollusca, 

 and in the common Slugs (Limacidae) and Snails (Helicidae) 

 these proceedings may be prolonged and complicated. 



Habits. 



The Mollusca are characteristically slow-moving and ' slug- 

 gish", but nearly all have some power of locomotion, by swimming, 

 creeping, or burrowing. The Cephalopoda are active and 

 powerful swimmers. 



The number of forms which are actually fixed down is very 

 small. A good many Lamellibranchs may be temporarily 

 anchored, but some, like the Oyster, are permanently attached 

 by the shell. 



The muscular strength of some Molluscs is known to be very 

 great. The Common Limpet, for example, can resist a pull of 

 G2 1b. 



The food of Mollusca and their methods of feeding are very 

 varied. Where a radula is present it is generally used for rasping 

 off portions of plant or animal tissues, but the Lamellibranchia 

 and a few Gastropoda feed upon minute floating life (plankton) 

 or on organic particles which are drawn in by the movements of 

 vibratile hairs (cilia) on the gills and mantle. Those molluscs 

 that are definitely carnivorous generally prey upon sessile 

 invertebrates such as the Coelenterates, Sponges, and Tunicates, 

 or upon other Mollusca, but the active Cephalopods pursue and 

 capture fishes and Crustacea. 



Many molluscs live in more or less close association with corals, 

 Sponges, and other animals, although cases of actual parasitism 

 are few. Some Gastropods are external parasites of Echinoderms, 

 while a few Gastropods and Lamellibranchs are highly degenerate 

 internal parasites of Holothurians. In their turn, molluscs are 

 the hosts of very many external and internal parasites. 



A remarkable instance of molluscs that utilize for their own 

 defence the organs of their prey is that of the Nudibranch 

 Gastropods of the family Eolididae. These feed on Hydroids 

 and other Coelenterates, and the stinging cells (nematocysts) of 

 the latter are stored up in the dorsal appendages of the Eolids, 

 from which they are expelled when the animal is attacked. 



Mollusca form the characteristic food of numerous other 

 animals. The Greenland Whale, which feeds upon plankton 

 organisms, swallows vast quantities of Pteropods. Other whales, 

 such as the Sperm Whale, live mainly on Cephalopods, and the 

 beaks and other parts of these molluscs may be found in their 



