BY A. J. JUKES-BROWNE. 
69 
very little, and observations directed to the elucidation of 
this point are much to be desired. 
If we assume that the edges of the mantle can produce a 
smooth surface or a sculptured one according to the animal’s 
requirements, we may perhaps also assume that a smooth 
shell is the normal product, formed when there is no 
special reason for it to be otherwise; for it is probable that 
the borders of the mantle are normally smooth, and that rugo¬ 
sities or prolongations are special modifications evolved 
mainly for protective purposes. Thus the foliaceous 
and frondose processes of the Murex shell may serve this 
purpose by imitating the fronds of seaweeds, and preserve 
the animal from the eyes of predatory carnivorous fish, 
but observations are needed to confirm this theory. 
The third great class of Mollusca are those which make 
their shelly covering in two pieces or valves, each one 
covering and protecting one side of the animal; hence 
they are known as the Bivalves or Conchifera. These 
animals have no definite head and no dental apparatus of 
any kind, but they have a sort of mouth and they feed on 
small animalculse brought to this mouth by the currents 
set up in the gills, which are supplied with water through 
two tubes called siphons. 
In this class the whole body is covered by a mantle in 
two lobes which secrete the two valves of the shell; this 
mantle is sometimes open below, sometimes closed, but 
there is always an opening for the extrusion of the foot, 
a soft, muscular, elongate, and extensible organ, which is 
used for the purpose of propulsion. Further, the hinder 
part of the mantle is always converted into two tubes or 
siphons, one being the incurrent and the other an ex¬ 
current siphon. These siphons are sometimes short and 
sometimes very long, but are always strong, muscular, and 
contractile; in some cases they are separate from one 
another, while in others they are partially or wholly united, 
and in some genera they are both enclosed in a papery kind 
of epidermis. 
The foot varies much in shape and length, being some¬ 
times very small and degenerate, more usually it is tongue¬ 
shaped, and sometimes it is much prolonged and curved, 
as in the Cockles ( Gardium ), enabling them to jump several 
inches at a time and to burrow quickly into soft sand. 
The margins of the mantle are generally smooth, but are 
sometimes frilled and even fringed. It is on these organs, 
the mantle, foot, and siphons, that observations are wanted 
