METAZOA ARACHNOZOA. 361 



ARACHNOZOA. 



Section 14 (in part). 



Trilobita. The trilobites and the king or horseshoe 

 crabs with their allies, the Arachnida, form a group inter- 

 mediate between the Crustacea and Myriopoda. Trilo- 

 bites are primitive in structure and offer good illustrations 

 of generalized segmented animals bearing jointed append- 

 ages. They also constitute one of the few groups which 

 well illustrate a natural classification. As Beecher * has 

 pointed out, the principles of such a classification can be 

 best applied in a group of animals which has a geological 

 history more or less complete, and which is not wholly 

 parasitic or greatly reduced. 



The trilobites have a long geological history covering 

 the time from the pre-Cambrian to the Permian. Their 

 structure is generalized and quite uniform, and no sessile, 

 stalked, parasitic, fresh-water, or land species is known. 

 We are therefore dealing with primitive, free-swimming, 

 marine forms. 



The stage in trilobites corresponding to the protoconch 

 of Cephalopods and the protegulum of Brachiopods is 

 known as the protaspis (PI. 887, fig. I, Sao hirsuta Bar- 

 rande). At this time the trilobite consists almost wholly 

 of the head region or cephalon, covered by a dorsal 

 shield and with a central axis clearly defined. The ab- 

 dominal segments which consolidated are called the 

 caudal shield or pygidium are now only indistinctly 

 outlined. The free cheeks are situated on the ventral 

 side and therefore cannot be seen in a dorsal view ; only 

 the eye-lines which in older stages extend from the central 

 axis to the eyes are now visible from above (see fig. i). 



1 Amer. Journ. Sci., (4), III, March, 1897, pp. 97, 98. 



