TEREDO. 153 



out the substance of the egg, and ultimately separate 

 into two nearly equal parts, one of which encroaches 

 by degrees on the other and at last completely enve- 

 lopes it. Towards the eleventh hour the yelk is trans- 

 formed into an agglomerative mass, composed of two 

 well-defined portions, and covered by a more or less 

 folded membrane. One of these portions now separates 

 into three lobes ; and vibratory cilia make their appear- 

 ance, at first short, thick, and few in number, after- 

 wards longer, finer, and much more numerous. The 

 cilia surround the entire body of the fry, which soon 

 swims with great rapidity, like one of the Infusoria. 

 This state lasts till nearly the forty-eighth hour : then the 

 number of the cilia diminishes, and the fry falls to the 

 bottom of the vessel, where it moves rather slowly. 

 At the same time the yelk- membrane is divided into 

 two equal parts. These are the rudiments of the shell, 

 which at first is quite membranous, flexible, and irre- 

 gularly oval, with a salient angle at the point cor- 

 responding with the hinge. In a short time this form 

 is altered ; the salient angle is effaced, and superseded 

 by a re-entering angle. The shell is then symmetrical 

 and heart-shaped, and at the same time is encrusted 

 by calcareous salts and solidified. During the forma- 

 tion of the shell the mantle is developed, with delicately 

 ciliated edges, which are destined to replace the original 

 ciliary apparatus. The new cilia are extensible and re- 

 tractile, and consist of a single row. The fry can 

 withdraw entirely into their shells. At this stage they 

 appear not to be sensible of noises, nor even of an 

 agitation of the water in which they are placed. It 

 constitutes a critical period of their lives ; and a large 

 proportion of the infantile community then perish. 

 About the sixty-eighth hour from the production of the 



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