430 MARINE INVERTEBRATES 
have become united at their ends to the mantle (on the outer gills) 
and to the body-mass (on the inner gills). Here also the first traces 
of the higher type of gill found in the next order appear ; that is, a 
gill in which the filaments are connected by vascular channels in- 
stead of by cilia. This order therefore is presumed to connect 
the true filamentous type of gill with the fully developed lamellar 
type. As the filaments do not always fuse with the mantle or 
foot, the description of the pseudolamellibranch gill becomes 
somewhat vague, and there is some room for doubt whether this 
order should exist at all. 
Most of the genera placed in it are monomyarian ; that is, they 
have but one adductor muscle, or if two, the anterior one is very 
small and unimportant. The mantle-edges are entirely open, 
there being no regularly formed siphons. As a rule, the foot is 
exceedingly small or quite aborted. 
FaMILy AVICULIDE 
In this family there is an exception to the rule.just stated: the 
foot is long. There is a byssus, and the gills (by their outer 
ascending filaments) are concrescent with the mantle. 
The interior of all the Aviculide is of a highly polished nacre, 
and some of the larger species have been extensively collected for 
mother-of-pearl. To the Aviculide belong the pearl-oysters 
(Meleagrina margaritifera). 
A delightful air of romance and adventure surrounds the 
pearl-fisheries, about which much has been written. The pearl isan 
abnormal product formed by the deposition of nacre around some 
foreign object, such as a grain of sand, or—usually, it is said— 
an egg which has failed to develop properly. They are seldom 
found in individuals having favorable conditions of growth, but 
those which are greatly crowded together and covered with va- 
rious parasites and growths are most productive of them. The 
pearl is the same in substance as the nacreous shell of the pearl- 
oyster, and its iridescence is due to the fine striz of the undulat- 
ing layers of which it is composed. The pearl as well as the shell 
is produced by the shell-glands of the mantle. The value of the 
