NO. 3 INSECT IIKAD SNODGRASS 29 



eyes (£). and the laliruni (Lin) ; but to it is attached the tergum of 

 the mandibular segment (I\ ) bearing the large, jaw-like mandibles 

 (Md). Following the mandibular segment, comes the region of the 

 two maxillar}' segments (V + VI) with the rudimentary first and 

 second maxillae on its under surface. Euhranchiptis thus re|)resents a 

 stage in the evolution of the head almost equivalent to that in the em- 

 bryonic development of insects shown in figure i6 C where the gnathal 

 segments {Gnc), in process of being united with the protocephalon 

 (Pre), still constitute a distinct body region. In Limnaclia (Choncos- 

 traca), the structure of the head is essentially as in Euhranchipus, but 

 the gnathal segments are more intimately united with the proto- 

 cephalon, and the second antennae are typical biramous appendages. 



In A pus (Notostraca) the head is more highly evolved (fig. 17 D. 

 E), and its lateral and posterior margins are produced into a large 

 cephalic carapace (Cp). The protocephalon and the gnathal segments 

 are imited, but their respective areas are well defined dorsally (D). 

 The protocephalon (Pre) is set ofif from the mandibular tergum (IV) 

 by a sinuous transverse groove (x) ; on its upj^er surface it bears 

 the group of head sense organs, including the compound eyes (E) , 

 and, on its lower surface (E), the antennae (Ant) and the labrum 

 (Lin). The tergal region of the mandibular segment (D, /F^) is dis- 

 tinctly limited posteriorly by a second suture ( \' ) on the dorsal sur- 

 face of the carapace, behind which is a narrow area representing the 

 dorsal wall of the two maxillary segments (['" + VI), from the pos- 

 terior edge of which is reflected the median part of the carapace. 

 Back of the head, and partly covered by the carapace, is the long, 

 flexible body of forty or more segments. Here is a condition f|uite 

 different, therefore, from that of the decapods (fig. 17B. C), in 

 which latter the protocephalon has retained its individuality, while 

 the gnathal segments have united with those of the maxillipeds and 

 the ambulatory limbs to form the region of the carapace (C, Cp). 



In the Amphipoda and the Isopoda, the head consists of the pro- 

 tocephalon, the three gnathal segments, and one or two of the maxilli- 

 ped segments. In these groups, however, the head segments are fused 

 into a cranium-like capsule (figs. 17 F, H, 28 A), in which little nr no 

 trace of the original head segmentation is to be discovered. In form 

 and general appearance, the amphipod head (fig. 17 H) often cu- 

 riously suggests the head of an insect, but both the amphipod and the 

 isopod cranium appears to contain at least one more segment than is 

 known to be included in either the insect or the myriapod head. 



The head in the Chilopoda (fig. 17 G), Diplopoda (K). and Hexa- 

 poda (I), is a highly evolved cranial capsule composed of the protoce- 



