and the Elements in which they live. 171 



already their value in the study of the lower tyj)es. I allude to 

 the relative bulk of animals of the same type living in different 

 media. We can derive no impression upon this point from the 

 investigation of the Radiata, as they are all aquatic, and almost 

 entirely marine. But the difference is already marked between 

 the Mollusca if we contrast their marine and their fluviatile and 

 terrestrial types within the limits of their natural secondary 

 groups. Among the Acephala, if we consider the Lamelli- 

 branchiata, we cannot fail to observe that the marine represen- 

 tatives are as a whole, and taking into consideration the propor- 

 tional number of their genera and species, of larger size and 

 greater weight than the fluviatile. AVe have nowhere such gi- 

 gantic, bulky and heavy freshwater bivalves, as are many of the 

 marine shells, and we need only compare the large Chamas or 

 Tridacnas and Hippopus, the gigantic Pinna, even with the 

 largest of Anodonts ; and again the numerous species of Cyclas, 

 &c., with the smaller marine bivalves, among which we find but 

 few species of so minute types. Again, among Gasteropoda how 

 much larger are most of the Univalve marine shells, such a» Do- 

 lium, Sti'ombus, Valuta and others, than even the largest fresh- 

 water AmpullariiB and the whole lot of freshwater and terrestrial 

 Pulmonata, among which latter we have absolutely the smallest 

 of all ]\Iollusca in the innumerable varieties of Pupa and other 

 genera ! We reckon in this type of Gasteropoda the minute spe- 

 cies by hundreds, while there are exceedingly few of really small 

 size among the marine ones, and the greater number are even 

 universally above the medium size of the larger fluviatile and 

 terrestrial types. 



Among the Articulata the same rule obtains, and here we may 

 compare classes with classes, even in their different stages of 

 growth. Are not the Worms, taken as a whole, larger animals 

 than the Caterpillars ? Do we not find among mai'ine Worms by 

 far the largest types ? We need only remember the gigantic 

 Eunice, or even the parasitic Tape Worms, to be satisfied of the 

 fact. Are not the Crustacea as a class composed of types exceed- 

 ing far the largest of Insects even with their wings spread ? Are 

 not the marine Lobsters many times larger than the freshwater 

 Crawfishes ? A minute investigation of the details of this nu- 

 merous class might lead to very intei*esting comparisons, which 

 however would be out of the way in this general sketch. 



I shall mention only a few facts to show that these comparisons 

 might even be traced between the different stages of growth of 

 these animals. It must be, for instance, a matter of surprise to 

 see that the body of so many Insects is smaller in their perfect 

 state of development than as a pupa ; and that again this is 

 smaller than that of the larva, though the larva be after all only 



12* 



