THE CRUSTACEA 



stood), where they formed, with the Arachnida, one of the divisions 

 of the Order Aptera. Three genera were recognised, Cancer, 

 3Ionoculus, and Oniscus. The Barnacles, however, forming the genus 

 Lepas, were placed among the Venues testacea, between Cliiton and 

 Fholas, and the genus Lernaea, comprising certain parasitic forms, 

 was classed under Vermes laollusca. The adjective cmdata or 

 Crustacea had been applied, more or less loosely, by the older 

 naturalists to animals possessing a hard exoskeleton, and Brisson, 

 in 1756, had used it as the designation of a group. Pennant, in 

 1777, appears to be the first post-Linnaean author to suggest the 

 separation of a distinct class under the name Crustacea, and this 

 step was definitely taken in Cuvier's Le(;ons d'Aiiafomie compan'e 

 (18U0), where, however, the Isopoda still remained among the 

 Insects. Lamarck in 1801 included the Isopoda, and Latreille made 

 manj'- changes in the classification, the most important being the 

 division of the class into Malacostraca and Entoiiiostraca {Genera 

 Crust, et Insect., 1806). This arrangement, with the further 

 division of the Malacostraca into EdriophtJiahna and Podoplttlialma, 

 proposed by Leach in 1815, has been widely adopted down to the 

 present day. 



The researches of J. C. Savigny on the structure of the mouth- 

 parts in Insects and Crustacea (181 6) laid the foundations of modern 

 conceptions of Arthropod morphology. Among his immediate 

 successors in this line of research, perhaps the most prominent 

 names are those of V. Audouin and H. Milne-Edwards. The 

 Histoire Natwrelle des Crustaces of the last-named author (1834-1840) 

 marks an epoch in the history of Carcinology and is still indispens- 

 able to the student. It is curious that, even at this date, Milne- 

 Edwards did not include the Cirripedia in his survey of the 

 Crustacea, although J. Vaughan Thompson had already in 1830 

 described their larval stages and recognised their Crustacean affini- 

 ties. Apart from this omission, the limits of the Class adopted by 

 Milne-Edwards differ from those now generally accepted in includ- 

 ing the Pycnogonida, the Xiphosura, and the Trilobita. There can 

 be little doubt that the affinities of these groups are Avith the 

 Arachnida, though it is possible that the very primitive Trilobites 

 were also phyletically related to the Crustacea. 



It is impossible to summarise here the numerous changes in 

 classification introduced since the date of Milne-Edwards' work, but 

 it may be mentioned that the establishment of a phylogenetic basis 

 for classification is largely due to the numerous and important 

 works of C. Claus. 



Further notes on the historical development of the subject will 

 be given in the chapters dealing with the separate sub-classes and 

 orders. 



