BIVALVE MOLLUSCA. 329 
animal and enables it to breathe. It is discharged by the second 
tube, when deprived of its oxygen and no longer respirable, carrying 
with it also 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 contracts 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 it is quite plain. On 
the upper edge of the anterior portion 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 intestine. 
The heart consists of two auricles and a ventricle, which beat at 
very irregular intervals, four or five in the minute. The blood is 
colourless, transparent, and charged with small irregular corpuscles. 
The act of breathing is accomplished in the branchie, or gills. 
Nevertheless, the one half of the blood returns to the heart without 
passing through these branchie. 
The nervous system is well developed, and consists of nervous 
filaments, and of ganglions, which are distributed to the mantle, the 
branchize, the foot, and the siphon tubes. 
The adult animal is surrounded by a sort of sheath, consisting of 
a solid shelly coat, which has sometimes been described, erroneously, 
as forming part of the animal. The Teredo, shut up in this tube, is 
limited in its movements ; when observed in a vase, its motions are 
slow and deliberate—movements of extension and contraction, by 
the aid of which it contrives with difficulty to exchange its place ; 
but nothing indicates a true creeping movement. In a state of 
nature, according to M. Quatrefages, the body of the animal is 
stretched out to three times its length without diminishing in any 
respect its proportional thickness; the afflux of water penetrating 
under the mantle, and of the blood which accumulates in the interior 
vessels, sufficiently accounting for a phenomenon which at the first 
glance is very singular. 
The Teredo lays a spherical greenish-yellow egg. Shortly after 
fecundation, these eggs are hatched. At first naked and motionless, 
these larve are soon covered with vibratile cilia, when they begin to 
move, at first by a revolving pirouette, afterwards swimming about 
freely in the water. When one of these larvee has found a piece of 
submerged wood, without which it probably could not live, the 
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