30 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



phalon and the gnathal segments, but so thoroughly fused are all the 

 cephalic elements that the segmental composition of the head is no 

 longer discernible in the head wall. The insect head is a well-standard- 

 ized structure, which, though varying greatly with regard to form, 

 is the same in fundamental construction throughout all the insect 

 orders. The myriapod head, likewise, exhibits no modifications in its 

 basic structure, and, from a study of the head alone, it is impossible 

 to judge whether the cephalic structures of the myriapods and of the 

 insects has had a common origin, or whether in each group the head 

 has been evolved along a separate line of development. Considering 

 the dififerences in the head appendages, and especially in the mandibles, 

 as will be shown later, it appears probable, however, that the myria- 

 pods and insects are not as closely related as the form of the head 

 might otherwise suggest. The insect head resembles also the head of 

 the amphipods and isopods, as already pointed out, but it can be 

 shown that the evolution of the head appendages has run parallel in 

 the insects and the crustaceans, and here again, therefore, we must 

 conclude that the similarities in the head structure are only equal 

 results of the primary tendency toward a condensation of the gnathal 

 segments with the protocephalon, in consequence of the drafting of 

 the appendages of these segments into the service of the mouth. 



Considering all the evidence, especially that which will be adduced 

 from a study of the mandibles, it seems most probable that the several 

 principal arthropod groups represent independent lines of descent 

 from ancestors differentiated at an early stage in the evolution of the 

 composite head structure. The early development of the thorax in 

 the insect embryo, before the gnathal segments are added to the head 

 (fig. 13), is evidence that the insects formed a distinct arthropod 

 group long before the completion of the definitive head, unless the 

 differentiation of the thorax in the young insect embryo is to be re- 

 garded as a precocious embryonic development, comparable with the 

 early development of the head in the vertebrate embryo. It is scarcely 

 necessary, however, to postulate, as suggested by Walton (1927), a 

 separate origin of the insects from annelids. 



In the Arachnida, the protocephalon constitutes a distinct head at 

 an early embryonic period, but, as shown in Balfour's illustration (fig. 

 22 C, Pre), it does not include at this stage the tritocerebral segment 

 (///) in its composition. At a later stage, however, the tritocerebral 

 segment and the five following segments are usually added to the 

 protocephalon to form a cephalothorax (fig. 17 J, CtJi). The appen- 

 dages of the cephalothorax of an adult arachnid are the chelicerae 



