202 



GASTEROPODA. 



day. Grant, Quatrefages, Dumortier, Leuckart, F. 

 Muller, Laurent, Sars, Van Beneden, Rathke, Loven, 

 Milne-Edwards, Nordmann, Kolliker, Gegenbaur, 

 Krohn, Clarapede, Vogt, and Lacaze-Duthiers are some 

 of those who have distinguished themselves by such 

 researches. All their observations show that the Gas- 

 teropoda pass through a series of metamorphoses before 

 attaining their perfect state, and that the duration of 

 the larval state is often considerable, compared with the 

 whole period of their existence. 



Their shells appear to have a more uniform structure 

 than those of the Acephala. Dr. Carpenter says " There 

 is not by any means the same amount of diversity in 

 the structure of the shell in the different subdivisions of 

 this group as that which we have met with among the 

 Conchiferous Acephala. There is a certain typical plan 

 of construction that seems common to by far the greater 

 number of them ; and any considerable departures from 

 it are uncommon. The small proportion of animal 

 matter contained in most of these shells is a very marked 

 feature in their character, and it serves to render other 

 features indistinct." A univalve shell consists of three 

 layers of cellular plates, each of the upper two layers 

 lying unconformably on the one immediately below it, 

 and every plate being composed of a single series of 

 elongated prismatic cells, which cohere lengthwise. He 

 dissents from the idea of Dr. Gray that the structural 

 arrangement is the result of crystalline action. The shells 

 of mollusca were formerly regarded as a mere exuda- 

 tion of calcareous matter, the particles of which were 

 held together by a sort of animal glue. Carpenter 

 is of opinion that the appearance of prismatic crystal- 

 lization in certain shells is entirely due to the moulding 

 of the calcareous matter within their cells. He agrees 



