188 SOLENOCONCHIA. 



pose, and are cleared out by swarms of Infusoria, which 

 appear to be generated from the corruption. In the first 

 stage of development the germ is motionless ; in the 

 second stage it is propelled by vibratile cilia, which are 

 set round a large lobe in front, similar to that observ- 

 able in the larvae of many mollusca, and it swims 

 rapidly; in the third stage it crawls by means of a disk- 

 like foot. In swimming it does not come to the sui'face 

 of the water, as do the fry of the oyster and other 

 moUusca. The shell is formed during the third period, 

 but is only detected by its iridescent lustre, being 

 exceedingly thin and transparent, a mere film. This state 

 continues till the fifth and occasionally the sixth day after 

 birth. The embryonic period lasts from thirty-five to 

 forty days. If any of the fry die, Paramecia and Plces- 

 conice (Infusoria) are bred from the decaying matter, 

 and, entering into the shells of living individuals, soon 

 destroy them. Lacaze-Duthiers observed a current of 

 water passing through the shell from the opening at the 

 smaller end. He discovered the Dentalium at low- water 

 mark, where its presence was betrayed by a small groove 

 in the sand ; and he seems to have got a knack of find- 

 ing them, for he says he easily procured 200 live speci- 

 mens at the recess of a single high spring tide. They pre- 

 fer certain spots, especially patches of coarse sand mixed 

 with broken shells and interspersed with Zoster a. In this 

 part of his researches he derived much assistance from the 

 hydrographical survey of France, the minute accuracy 

 of which he greatly praises, not merely as regards 

 zoology, but as subservient to the navigation of the 

 coast. I fear we cannot say so much for ourselves on 

 this side of the Channel, when we reflect on the shame- 

 ful delay that takes place in the publication of our charts, 

 and even now find that the hydrographical survey on 



