174 AFFINITIES AND GEOGRAPHICAL 



tlie former being the simpler, and exclusively aquatic. Em- 

 merich considers the Trilobites which figure so conspicuously 

 in the early rocks, as between the two divisions, but most 

 nearly allied to the first ; whence it would appear that the 

 Crustacea which make so early an appearance in the rock series, 

 are humble animals, only preceded in their own sub-kingdom 

 by a group which, from their slight forms, might be ill-adapted 

 for preservation in strata exposed after deposition to a high 

 temperature. The geological history of the Crustacea tallies in 

 other points with their gradation. In the triassic epoch come 

 the Macruri, which prevail to the present time ; afterwards, in 

 the tertiary era, come the Brachyura. These are the fossil 

 orders which have been best studied, and it is M. Agassiz who 

 says, " they succeed each other in the series of formations in 

 the order of their organic gradation." The same naturalist 

 remarks " the intimate analogy between these different types 

 and the phases of the embryonic development of the Crustacea, 

 which MM. Eathke and Erdl have afforded us the means of 

 becoming acquainted with." l As elsewhere observed, the young 

 of the decapoda are of the entomostracous form, and thus 

 denote a passage of the one from the other. 



In one family of the Crustacea, there is a striking illustration 

 of what is here set forth as the true history of species. This 

 is the family to which the well-known hermit-crabs (Paguri) 

 belong, distributed extensively in the tropical American islands, 

 and upon our own coasts. Animals of this kind live in mol- 

 luscan shells deserted by the proper tenants. They select one 

 at the first for their residence, and afterwards, as they increase 

 in size, they remove to larger ones. With the hind part of the 

 body inserted in the hollow shell, they present the head and 

 feet outwards. They move about in the shallow water, upon 

 the shore, and even upon dry land, with great freedom, drag- 

 ging their adopted mansion after them. A very slight exami- 



1 " Easting upon the characters derived from the nervous system, 

 which in the crabs is concentrated into a few masses, zoologists have 

 generally considered these animals as higher than the lobsters, in which 

 the nervous ganglia remain more isolated. Now, as far as we know, 

 the embryos of brachyuran Crustacea, that is, of crabs, are all macrural 

 in their shape ; that is to say, they resemble at an early age the lobsters 

 more than their own parents : and again, lobster-like Crustacea prevailed 

 in the middle ages of geological times, during the triassic and oolitic 

 periods, that is, before crabs were created, as we find no fossils of that 

 family before the tertiary period." Agassiz on Lake Superior, 1850, 

 p. 197. 



