324 THE OCEAN WORLD. 



The act of breathing is accomplished in the branchiae, or gills, and 

 mantle. Nevertheless, the one half of the blood returns to the heart 

 without passing through these branchiae. 



The nervous system is well developed, and consists of a brain, 

 nervous filaments, and of ganglions, which are distributed in the 

 mantle, the branchiae, and the syphon tubes. 



The adult animal is surrounded by a sort of sheath, consisting of a 

 solid mucus, which has sometimes been described erroneously as form- 

 ing 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 change 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 ac- 

 counting for a phenomenon which at the first glance is very singular. 



The Teredo deposits a spherical greenish-yellow egg. Shortly after 

 fecundation, these eggs are transformed into larvae. At first naked 

 and motionless, these larvae 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 larvae has 

 found a piece of submerged wood, without which it probably could 

 not live, the curious spectacle is observed of a being which fabricates, 

 step by step, and as it requires them, the organs necessary for the 

 performance of its functions. It begins by creeping along the surface 

 of the wood by means of the very long feet with which it is furnished. 

 Then it is observed from time to time to open and shut the valves of 

 the little embryo shell which partly envelopes it. As soon as it has 

 found a part of the wood sufficiently soft and porous for its purpose, it 

 pauses, attacks the ligneous substance, and soon produces a little pore, 

 or cell, which will be the entrance to the future canal. 



Once fairly lodged in this little cell, the young Teredo is rapidly 

 developed ; it covers itself with a coating of mucous matter, which, 

 condensing by degrees, assumes a brownish tint, forming a solid 

 covering, with two small holes for the passage of the syphon tubes. 

 At the end of three days this covering has become quite solid ; it is 



