DISTRIBUTION OF ORGANISMS. 155 



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 remarks " they succeed 

 each other in the series of formations in the order of their 

 organic gradation." The same naturalist remarks " the inti- 

 mate analogy between these different types and the phases of the 

 embryonic development of the Crustacea, which MM. Rathke 

 and Erdl have afforded us the means of becoming acquainted 

 with." As elsewhere remarked, 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 illustra- 

 tion of what I regard as the true history of species. This 

 is the family to which the well-known hermit-crab belong 

 (Paguri,) distributed extensively in the tropical American 

 islands, and upon our own coasts. Animals of this kind live 

 in molluscan shells deserted by their 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, dragging their adopted mansion after 

 them. A very slight examination of these animals shows 

 that they are adapted by special peculiarities for this kind of 

 life. In the common British hermit crab, the third and 

 fourth pairs of locomotive limbs are of small size, being 

 buried wholly within the shell, where they are applied to 

 the columellar fold, as a means of fastening the animal in the 

 recess. Farther in, and also employed in fastening the body 

 to the shell, is the caudal part, with two holders developed for 

 this express purpose, and as rough as a file. The hold is still 

 further secured in some species by rows of suckers along the 

 abdomen. Add to all this, that, for want of room at the 

 mouth of the shell, only one of the pincer claws is well deve- 

 loped, usually the right, while only the two front pairs of feet 

 are used for locomotion, and we see that, whether we take 



