132 PEOGRESSIVE STRUCTURE 



whole group of animals which produce what are properly 

 called shells, the favourite study of the conchologist ; 

 also a multitude which are shell-less at all periods of 

 their life ; and others, whose shells are reduced to mem- 

 branous plates concealed under the fleshy folds of their 

 bodies. Commencing with the shapeless bag of the 

 Ascidice; proceeding thence, through the bivalve shells 

 into Sea Slugs ; and so, through the various tribes of 

 univalve shells, we pass under review a great variety 

 of animals, rising in complication of structure one above 

 another, until we arrive at the Nautilus and the Cuttle- 

 fish, which close the great group, by a type of structure 

 in which the peculiar organization of the vertebrate is 

 dimly sketched. The lowest Mollusca are scarcely more 

 organized than the Zoophytes : the highest closely border 

 on the most perfect animals. The study of this class 

 therefore is, in all respects, highly important. To the 

 mere student of comparative anatomy it offers a rich 

 field of research ; for here, within circumscribed bounds, 

 he can trace the gradual development of organs from 

 the first idea, as it were, to their full perfection. To the 

 collector of beautiful objects, the countless varieties of 

 shells, so easily preserved and so varied in contour and 

 colour, afford continual sources of interest ; and their 

 proper classification, a pleasant problem for the exercise 

 of ingenuity : although it must be admitted that the 

 proper classification of shells cannot be arrived at, if 

 the nature of the animal which forms them be not 

 carefully studied. A striking proof of this is shown in 

 the genus Patella, of the older authors. If we merely 

 consider the form of the shell, this group appears to be 



