154 TEREDINID.E. 



egg the cilia commence growing, and become stronger. 

 The duration of the last period of growth is unknown. 

 Some of the fry survived for 130 hours. The perfect 

 larva swims rapidly, like a Rotifer, and has a long, nar- 

 row, and strap-shaped foot very flexible and resembling 

 that of a young mussel by means of which it creeps 

 with apparent ease along the bottom of the vessel. It 

 remains for a long time suspended in the water by a 

 transparent filament from the sides of the vessel. The 

 shell then becomes nearly globular, instead of irregu- 

 larly oval ; a pair of red eyes are seen in the middle of 

 the body ; and otolites, or ear-stones, and other organs 

 are formed. The eyes afterwards disappear, the body is 

 elongated, and the animal assumes its complete form. I 

 have given the above description of Quatrefages nearly 

 in full, because it explains the embryogeny of the Con- 

 chiferous Mollusca in general. This eminent zoologist 

 is of opinion that the Teredo undergoes a true or complete 

 metamorphosis. In the first state of growth its integu- 

 ments are membranous ; it has no distinct organs ; its 

 sole mode of locomotion is by means of cilia, which cover 

 the body : it is a larva. In the second state it has acquired 

 a shell ; it possesses distinct organs of sense, besides a 

 special apparatus for swimming and a foot for creeping : 

 it is then a chrysalis or pupa. The third and last state 

 represents an imago ; the transformation has been com- 

 pleted, and the animal thus developed enters upon a 

 new phase of life, with appliances peculiarly adapted 

 to its altered conditions. In reality, however, the evo- 

 lution from a simple globule into a shell-fish endowed 

 with a comparatively high degree of organization, and 

 of a complicated structure, is not the result of sudden 

 changes, but is effected by a series of successive growths, 

 or epigenesis. The outer membrane of the egg becomes 



