410 
THE OCEAN WORLD. 
Indian Ocean and Oceania. Some species, however, such as Bulla 
pulla (Figs. 206 and 207), the shell of which is shaded grey and brow 11 ; 
and the Water-drop (Bulla hydatida), inhabit European seas. Bud' 1 
oblonga and Bulla aspersa (Adams), and Bulla nebulosa (Gould); 
represented in Figs. 208, 209, and 210, are also well-known species. 
In the Pectinibranchial Gasteropods the gills are composed 
numerous leaflets cut like the teeth of a comb, and attached, on one 
or many lines, to the upper part of the respiratory cavity. They 
constitute the most numerous order of Cephalous Molluscs, comp re ' 
hending nearly all the univalve spiral shells, and many others which 
are simply conical. They inhabit the sea, rivers, and lakes, and are 
of all sizes. The most remarkable genera to which we shall li^ 
ourselves belong to the family of Troehoidm and Buccinoidw. 
TkOCHOID/E. 
The genus Troclms are found in all seas, and near to the shore 111 
the clefts of rocks, especially in places where seaweeds grow luxuri' 
antly. Some of these thick, cone-shaped shells are extremely beautiftd’ 
being richly nacred inside, and remarkable for the beauty and diver 
sity of colour exhibited. Generally smooth, the great spiral is, neve 1 ' 
Fig. 211. Trochus niloticus (Linnams). Fig. 212. Trochus virgatus (Gm^-) - 
theless, sometimes edged with a series of regular spines. The for# 1 
conical, the spiral more or less raised, broad and angular at the base , 
the opening entire, depressed transversely, and the edge disunited in 
the upper part. 
The animal which inhabits this shell is also spiral ; its head is f# 1 
