. Birds of other lands, reptiles, fishes, jointed animals and lower forms;. Zoology; Birds; Reptiles; Fishes. 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, th
. Birds of other lands, reptiles, fishes, jointed animals and lower forms;. Zoology; Birds; Reptiles; Fishes. 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 gill-tuft, observed by the writer on the West Australian reefs, was over lO inches long and 8 inches broad. Its general ground-colour was intense vermilion, relieved, however, by a frilled border nearl}-- 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 consequent!}' 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 ha\e a compactly disposed comb- like contour. This gill-tuft is situated in an exca\-ated chamber inside the shell, immediately over the neck. The Common Whelk, the 's-FOOT Shell, and the Winkle are a few typical British marine representatives of this group, which
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