STRUCTURE OF CONftHIFERA, OR BIVALVES. 119 



parts. But when any alarm or irritation causes the animal to 

 close its shell, it does so by means of a muscle (sometimes 



Fig. 61. SHELL op 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 the 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. JSear 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, 



