586 ZOOLOGY SECT. 



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. 

 Peracarida are known from Palaeozoic 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 zoaea-stage, many 

 of the lower forms progressing no further. But in Malacostraca 

 the zoaea 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 larvae 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 exclusive reference to their 

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



Many Crustacea present instances of protective and aggressive 

 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 (Paramithrax), 

 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 of Dromia, carries 

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

 another member of the same family the 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 

 association of Hermit-crabs with sea-anemones, has already been 

 referred to (p. 205) : another interesting example is the occurrence 

 of the little Pea-crab (Pinnotheres) in the mantle-cavity of Mussels. 

 Other Decapods are found in the intestines of Sea-urchins and 

 Holothurians, and one genus of Crab lives in a cavity in a Coral, 

 the aperture being only just sufficient to allow of a due supply 

 of food and water. 



It is in Crustacea that we find the first indication of characters 

 the purpose of which appears to be their attractiveness to the 

 opposite sex. The immensely enlarged and highly coloured chelae 

 of some male crabs (Gelasimus, Fig. 476, 2) are said to be used for 

 attracting the female as well as for fighting. The sound-producing 



