METAZOA CRUSTACEA. 325 



CRUSTACEA. 



Section 12 (in part), Section 13, and Section 14 



(in riart\ . 



(in part) 

 ENTOMOSTRACA. 



The Worms with the remaining large classes of Inverte- 

 brates the Crustacea, Arachnozoa, Myriopoda, and 

 Insecta were formerly grouped together under the name 

 of the Articulata or animals with articulated bodies. 

 Afterwards the Worms were placed in the subkingdom of 

 Vermes, and the other classes, taken collectively, were 

 known as the Arthropoda or animals with jointed legs. 

 Of late years there has been much discussion on the 

 question, " Do the Arthropoda constitute a natural group ?" 

 That is, have these different classes descended from a 

 common ancestor ? The tendency is toward a negative 

 answer, and the giving up of the name Arthropoda, while 

 the classes are considered separately, each having its own 

 ancestral fojrns. This course we have followed in the 

 Guide. 



The trilobites of Cambrian times may be ancestral 

 forms of the Crustacea. They are certainly a generalized 

 group, possessing characters in common not only with the 

 Crustacea but also with the Arachnozoa. This being the 

 case, it may be that some pre-Cambrian trunk form gave 

 rise to both groups which developed and spread out 

 into separate branches reaching down to the present day, 

 while the parent group the Trilobita became extinct 

 in Palaeozoic times. 1 



The relationships, however, between the trilobites and 



'See Woodward, Quart. Journ. Geol. Soc. London, LI, 1895, 

 p. Ixxi. 



