344 THE LIVING ANIMALS OF THE WORLD 



notable for the slug-like form of their body, which is usually supplemented by the out- 

 growth from it of complex, variously modified gill-filaments. In some species these exter- 

 nal gills take the form of symmetrical flower-like tufts at the posterior end of the back, 

 while in others simple or variously branched gills may be developed on the upper-surface. 

 The colours of many of these sea-slugs are more brilliant than those of any other molluscs, 

 this being especially the case with the tropical coral-reef-frequenting species. Bright scarlet, 

 yellows, and blues, separately or variously combined, are among the dominant tints. Many 

 of these tropical species are also of considerable size. One particular kind, having a flower- 

 like dorsal (^ill-tuft, observed by the writer on the West Australian reefs, was over lo inches 

 Ion" and 8 inches broad. Its general ground-colour was intense vermilion, relieved, however, 

 by a frilled border nearly an inch in width of the purest white, with radiating streaks of 

 scarlet. It is an interesting circumstance that these naked-gilled molluscs, shell-less so far 



as their adult phases are concerned, emerge 

 from the egg with a perfectly formed, but neces- 

 sarily very minute, transparent shell, resembling 

 that of a garden-snail. It is consequentl}' inferred 

 that the group has been derived from some per- 

 manently shell-bearing form. 



The Comb-gilled section embraces the great 

 majority of the marine molluscs having a single 

 more or less convoluted or spirally twisted shell. 

 They take their name from the circumstance 

 that the gills have a compactly disposed comb- 

 like contour. This gill-tuft is situated in an 

 excavated chamber inside the shell, immediately 

 over the neck. The Common Whelk, the 

 I'EIJCAN'S-FOOT SllEl.L, and the Winkle are a 

 few typical British marine representatives of this 

 group, which, however, attains to the zenith of 

 its development in the size, variety of form, and 

 ornate coloration of its shells in tropical seas. 

 The inter-tropical coral-reefs in particular yield 

 a most abundant harvest in this direction. The 

 shells in common use obtained from such a 

 source include the ponderous Helmet-SHELL.S, or 

 CoNCHS, employed for the manufacture of cameos ; 

 the Gi.'kNT Whelks and Trumpet-shells, often 

 over 1 8 inches long, used as signal-horns through- 

 out Polynesia and on the tropical Australian 

 coast ; and the capacious Melg.V-shells, made 

 to do dut\' for boat-baling and as water-vessels and general domestic receptacles throughout 

 the same tropical area. To this list may be added the Harp-SHELLS, VoLUTES, Cones,. 

 Mitres, Olives, Thorny Woodcocks, and a host of others prized by the conchologist. 

 To this .section must also be referred the innumerable species of COWRIES, of which the large, 

 boldly mottled "Ti(;er " and " Faxther" species are well known. The comparatively small, 

 yellowish, thickly built, porcelain-like shell of the " Money-cowrie " constitutes, as is well 

 known, the current coin throughout extensive areas of Africa and India. It is recorded that as 

 large a quantity as sixty tons of these small shells, originally collected from tropical seas, have 

 been shipped from one British port alone to the African coast for commercial use within a 

 single year. One very diminutive cowrie, pale pink in colour, with a delicately streaked surface, 

 is indigenous to British waters. 



The third large group of Molluscs which demands attention is that of the Bivalves, or 



Fhtu by W. SavillfKt 



\_Miir,rd-on-Si, 



F.Z.S.-[ 



HIGHLY MAGNIFIED TONGUE 

 OF A SEA-SNAIL 



IVsth this structutf its o'zuner bores or files a kcle through 

 the shells of other molluscs upon ivhich it preys 



