BIVALVE MOLLUSC A. 323 



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 

 Avith four labial palpi, a stomach without any pecuhar 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 branchiae, or gills. 

 Nevertheless, the one half of the blood returns to the heart without 

 passing through these branchiee. 



The nervous system is well developed, and consists of nervous 

 filaments, and of ganglions, which are distributed to the mantle, the 

 branchiae, 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 Q.gg. Shortly after 

 fecundation, these eggs are hatched. At first naked and motionless, 

 these larvae are soon covered with vibratile cilia, when they begin to 

 move, at first by a revolving pirouette, aftenvards swimming about 

 freely in the water. When one of these larvae has found a piece of 

 submerged wood, without which it probably could not live, the 



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