NO. 3 INSECT HEAD — SNODGRASS 2J 



niyriapods and insects they have become reduced to rudiments, or 

 to embryonic vestiges. 



Insects were thoroughly modern in the later part of the Carbon- 

 iferous period, when their remains are first known from the geological 

 records. They must have been in the course of evolution during all 

 the preceding extent of the Paleozoic era. Scorpions are found in 

 the Silurian rocks, eurypterids in the Ordovician. Crustaceans, as 

 represented by trilobites and other forms, were well developed in the 

 Cambrian. The common arthropod ancestors in the protocephalic 

 stage, long antedating the divergence of the several modern groups, 

 must have existed, therefore, in remote ages of Pre-Cambrian time. 



THE DEFINITIVE ARTHROPOD HEAD 



In all modern arthropods, at least one pair, and usually several 

 pairs of the segmental appendages following the protocephalon are 

 modified to form organs of feeding, and they are crowded forward 

 toward the mouth, those of the first pair coming to lie at the sides 

 of the mouth opening. These appendages become the " mouth parts " 

 of insects, and in general they may be termed the gnathal appendages. 

 As a consequence of the forward transposition of the gnathal appen- 

 dages, the postoral, sternal parts of the protocephalic segments are 

 reduced and in most cases practically obliterated, their places being 

 taken by the sterna of the gnathal segments. Early in the course of 

 evolution, therefore, the gnathal segments themselves must have had 

 a tendency to fuse with the protocephalon to form an enlarged head 

 region ; and nearly all the arthropods show in some degree the results 

 of this tendency toward a more extensive cephalization of the anterior 

 segments in the formation of a composite definitive head. 



The condensation of the anterior segments has resulted in the for- 

 mation of a definite cephalic structure in many of the arthropod 

 groups. Among the Crustacea, however, there is much variation in 

 the composition of the head. In the decapods, the protocephalon alone 

 forms a distinct though immovable head piece — it is that part attached 

 within the anterior end of the carapace, overhung by the rostrum, that 

 bears the eyes, the antennules, the antennae, and the labrum, and 

 which may be easily detached from the region covered by the cara- 

 pace (fig. i/B). The segments of the mandibles, the maxillae, the 

 maxillipeds, and the legs are united dorsally in the wall of the cara- 

 pace. The jaws of the decapods, therefore, are not attached to the 

 primitive head, and though the protocephalon and carapace may be 

 said to constitute a " cephalothorax," there appears to be no reason 



