STRUCTURE OF CONCHIFERA, OR BIVALVES. 119 
But when any alarm or irritation causes the animal to 
close its shell, it does so by means of a muscle (sometimes 
Pig. 61.—SHELL oF TRIDACNE. 
single, sometimes double), which stretches across from one 
valve to the other, and which, by contracting, draws them 
together. Each valve is lined by an extended fold or lobe 
of thé mantle. In the higher tribes of the class, these lobes 
are united along their edges, leaving apertures for the ingress 
and egress of water (which are sometimes prolonged into 
tubes, fig. 150), and another for the foot. But in the Oyster 
and its allies, which have no foot, or a very small one, the 
mantle-lobes are quite disunited. The accompanying diagram 
(fig. 62) gives a general idea of the arrangement of the 
organs in one of the higher acephalous Mollusca, the Mactra, 
which is among those having two muscles for the drawing 
together of the valves. The upper end, as represented in 
this figure, is that which is considered as the anterior end or 
front of the animal, being that nearest which the mouth lies; 
and the posterior extremity (the lowest in the figure) is that 
at which the intestinal canal terminates, and at which the 
respiratory tubes are formed. Near the anterior muscle, we 
find the mouth, or entrance to the stomach ; it is furnished 
with four riband-shaped tentacula, of which one is seen in 
the figure ; and these seem to possess peculiar sensitiveness. 
Near the mouth lie the anterior ganglia of the nervous 
system, which represent the brain of higher animals; and 
these are connected by long cords with the posterior ganglion, 
which lies near the posterior muscle. The stomach, intes- 
tines, and liver occupy the central portion of the cavity of 
the shell ; and the intestinal tube is seen to pass backwards, 
