GENERAL NOTES ON MOLLUSCS. 415 
the common limpet (fate//a) (Fig. 225), as well as in 
terrestrial forms like the snail, where the mantle cavity 
forms the pulmonary chamber. Even in Lamellibranchs, 
where the gills are present in much modified form, it is 
probable that the mantle has much importance in respira- 
tion, the gills being perhaps of most importance in connec- 
tion with nutrition, and as brood-chambers. In those 
Gasteropods in which the gills are suppressed, there are 
often special respiratory organs (“adaptive gills”), such as 
the circle of plumes around the anus in Doris and its allies 
(Fig. 224). The osphradia are 
absent in Cephalopods, except 
in Nautilus, and one at least 
is usually suppressed in Gas- 
teropods. 
Shell.—On the dorsal surface 
of almost every mollusc em- 
bryo there is a little shell-sac 
in which an embryonic shell is 
begun; the adult shell, how- 
ever, is always started and 
increased by the mantle. Like 
other cuticular products, it has 
an organic basis (conchiolin or - deed , 
conchin), along | with which Pie, ant 
carbonate of lime is associated. 3,4 Hatley. 
There is a thin outer “horny 7 Note simple eyes at base of tentacles, 
layer, a, thick median “pris- ~ mouth, median foot, and vascular 
matic” see oi a Seale. mantle replacing the 
an internal mother-of-pear 
layer, which may be divided into two strata by a clear 
intermediate layer, well seen in the fresh-water mussel, 
Margaritana margaritifera. 
My. Irvine’s experiments at Granton Marine Station suggest that the 
lime salt originally absorbed is not the carbonate (of which there is a 
scant supply in sea-water), but the sulphate (which is abundant), and 
that the internal transformation from sulphate to carbonate is perhaps 
associated with the diffuse decomposition of nitrogenous waste products. 
Thus carbonate of ammonia, which seems to occur abundantly in the 
mantle of perfectly fresh mussels, would, with calcium sulphate, yield 
carbonate of lime and ammonium sulphate. One cannot suppose that 
shell-making is expressible in a chemical reaction of this simplicity, but it 
