PHOLADID^E. 



323 



which can be regarded as a foot. Starting from this point, all the 

 body of the Teredo is enveloped by the shell and mantle, which form 

 a sort of sheath communicating by two syphons with the exterior. 



The mantle adheres to the circumference of the 

 shell. Above, it forms two great folds, which may 

 both be swollen by the afflux of the blood, and acquire 

 considerable size. One of these folds placed in ad- 

 vance, which is called the cephalic hood, is worthy of 

 attention. The tissue of the mantle is of a greyish 

 tint, very light, and transparent enough, especially 

 in the young, to permit of the mass of liver, the 

 ovary, the branchiae, and the heart being distin- 

 guished in the interior, even to counting its pulsations. 

 The syphons are extensible, and attached the one to 

 the other for about two-thirds of their length, the 

 upper part being longer and thinner than the lower. 

 It is by these tubes that the aerated water enters 

 which feeds and enables the animal to 'breathe. It is 

 discharged by the second tube, when deprived of its 

 oxygen, and no longer respirable, carrying with it 

 the useless products of digestion. This movement is 

 continuous ; but from time to time the animal shuts 

 at once the orifices of both tubes, and slightly con- 

 tracts itself. 



The shell, seen on the side, presents an irregularly 

 triangular form ; it is nearly as broad as it is long ; 

 its two valves are solidly attached the one to the 

 other, above and below, by the mantle in such a 

 manner as only to permit of very slight movements. 

 It is coloured in yellow and brown lines : sometimes 



... . , . * ~ , ' Fig. 129. Teredo navalis 



it is quite plain. On the upper edge of the an- (Linnaeus), 

 terior truncature of the body of the animal is the mouth, a sort of 

 funnel, flat and slightly bell-shaped, furnished with four labial palpi, 

 a stomach without any peculiar feature, and a well-developed intes- 

 tine. 



The heart consists of two auricles and a ventricle, wh^ch beat at 

 very irregular intervals, four or five in the minute. The blood is 

 colourless, transparent, and charged with small irregular corpuscles. 



Y 2 



