CHAP. XV Backboneless Animals 247 



fashion, and may be something like the original mollusc. Whence 

 that original sprang is uncertain, but the common occurrence of the 

 trochosphere larva and some of the characters of the primitive 

 Gasteropods {Neortunia^ Chatodernia, Chiton) suggest the origin 

 of molluscs from some " worm " type or other. We can be sure ol 

 this, however, that the series must have divided at a very early 

 epoch into two sets, the sluggish, sedentary, headless bivalves on 

 the one hand, and the more active and aggressive snails and cuttle- 

 fish on the other. 



Relation to Man. — Irresistibly we think first of oysters, which 

 Huxley describes as "gustatory flashes of summer lightning," 

 and over which neolithic man smacked his lips. But many others, 

 cuttlefish, ear -shells (Haliotis). mussels {Mytilus edulis), peri- 

 winkles (Lit tori )ia littorea), cockles ( Ca7-dium cordatum), etc. etc. , are 

 used as food, and many more as bait/ In ancient days, as even 

 now, the shells of many were used for ornaments, instruments, 

 lamps, vessels, coins, etc. ; the inner layer of the shell furnishes 

 mother-of-pearl ; concretions around irritating particles become 

 pearls in the pearl-oyster {Alargariiand) and in a few others ; the 

 Tyrian purple was a secretion of the whelk Purpura and the 

 related Murex ; and the attaching byssus threads of the large bivalve 

 Pitina may be woven like silk. 



On the other hand, a few cuttlefish are large enough to be 

 somewhat dangerous ; the bivalve Teredo boring into ship-bottoms 

 and piers is a formidable pest, baulked, however, by the pre- 

 valent use of metal sheathing ; the snails and slugs are even more 

 voracious than the birds which decimate them. 



Conchology was for a while a craze, rare shells have changed 

 hands at the cost of hundreds of pounds, such is the human " mania 

 of owning things." But the shells are often fascinating in their 

 beauty, and poetic fancy has played lovingly with such as the 

 Nautilus. 



