SYSTEM OF NATURE. 
85 
correspondence, we can only seek for correspondence in 
economy. In this spirit I have pointed out the con¬ 
nexion between bats and birds, anteaters and reptiles, 
whales and fishes; and in this spirit I shall endeavour to 
trace a certain but less distinct connexion between these 
abnormal vertebrates and those vast groups of inverte¬ 
brates by which I suppose them to be surrounded. 
1st. Saltantia. The Crustacea appear to me to occupy 
the highest rank among the invertebrate classes : their cir¬ 
culation is distinct, double, and of a much higher charac¬ 
ter than that of insects; their respiration is normally by 
branchiae, but in some of the tribes by tracheae ; the brain 
is concentrated in the higher orders, and is very distinct; 
the senses of sight, smell, taste and hearing, appear to be 
also very distinct. There are two peculiarities connected 
with the generation of Crustacea that are worthy of much 
attention ; the first is that the young ones or eggs after 
extrusion are transferred to the surface of the abdomen, 
where they remain frequently for weeks, increasing slowly 
in size : the second peculiarity is, that when these eggs 
first become independent beings, possessing an existence 
and locomotive power distinct from that of the parent, 
they assume a form different from that of the parent ani¬ 
mal. For this discovery, one of the greatest of which 
modern Zoology can boast, we are indebted to Mr. J. V. 
Thompson. I think, after the detailed account given of 
the mode of reproduction in the marsupial animals, its 
similarity to that of macrourous Crustacea cannot fail to 
strike every reader. The marsupials are the highest of 
implacental vertebrates, the Crustacea highest of inverte¬ 
brate animals, and their most striking peculiarity — that 
of their anomalous reproduction — is strictly identical. 
