xi PHYLUM ARTHROPODA 555 



solitary ; others, like Shrimps, are gregarious, occurring in immense 

 shoals. Most of them either prey on living animals or devour 

 carrion, but, as we have seen, the barnacles are fixed, and feed on 

 minute particles after the fashion of many of the lower animals, 

 and the members of more than one order are parasites remark- 

 able for their deviation from the typical structure of the class 

 and their adaptation to their peculiar mode of life. In si/c 

 they present almost every gradation from microscopic Water-fleas 

 to Crabs two feet across the carapace, or four feet from tip to 

 tip of legs. 



As to geographical distribution, all the chief groups are cosmo- 

 politan, and it is only among the families, genera, and species that 

 matters of interest from this point of view are met with. Fossil 

 remains are known from very ancient periods. The oldest forms 

 are usually referred to the Phyllocarida, and occur from the Cam- 

 brian to the Trias. The shells of Ostracoda are also known from 

 the Cambrian upwards, and those of Cirripedia from the Silurian. 

 Arthrostraca are known from palceozoic times, but are rare as 

 fossils: the earliest Macruran is a shrimp-like form from the 

 Devonian, while the highly differentiated Brachyura are not 

 known with absolute certainty until the Cretaceous period. 



It was in the Crustacea that the recapitulation theory so often 

 alluded to was first worked out in detail. Embryology shows 

 that all Crustacea may be traced back in individual development 

 to the nauplius, upon which follows some kind of zoa3a-stage, many 

 of the Entomostraca progressing no further. But in Malacostraca 

 the zoa?a is followed by the mysis-stage, which is permanent in 

 Schizopods, transient in Decapods. It was certainly a tempting 

 hypothesis that this series of forms represented as many ancestral 

 stages in the evolution of the class. But we have to remember 

 that all such free larvre are subject to the action of the struggle 

 for existence, and have no doubt been modified in accordance with 

 their own special needs and without reference either to their 

 ancestors or to the adult species, into which they finally change. 



Many Crustacea present instances of protcdicc and aygrcs- 

 .s//v characters, i.e., modifications in form, colour, &c., which serve 

 to conceal them from their enemies or from their prey. Probably 

 the most striking example is that of certain crabs (Pqramithraso), 

 which deliberately plant Sea- weeds, Sponges, Alcyonarians, Zoo- 

 phytes, &c., all over the carapace, and are thus perfectly concealed 

 except when in motion. Another Crab, a species ofiDromia, carries 

 a relatively immense Ascidian or Sea-squirt on its back, and in 

 another species of the same genus tLe hinder legs are used to 

 hold umbrella-wise over the back a single valve of a bivalve 

 shell. 



Several instances of commensalism occur in the class. The 



