NO. I 



THE INSECT HEAD SNODGRASS 



first segment following. In a stomatopod the second antennae are 

 membranously connected with the protocephalon, in some other 

 crustaceans they arise just within the posterior sclerotized margin of 

 the head, and in an amphipod they have almost come together on the 

 midline of the face. Certainly in the amphipod the second antennae 

 must have migrated from their own segment into the blastocephalic 



2Mx 

 2Arit Md" iMx J\. 



2Anb Lm Q 



Fig. 2. — Examples of simple crustacean heads. 



A, Nebalia bipes, anterior end of body, gnathal region opened on left side. 

 B, Same, free head lobe, dorsal. C, Eubranchipus vernalis, protocephalon 

 (Prtc) and anterior trunk segments, mandibular segment (//) not united with 

 head. D, Callinectes sapidus, protocephalon, dorsal. 



part of the head. The crustacean protocephalon, therefore, evidently 

 represents the embryonic head lobe which has secondarily taken over 

 the second antennae, while the segment of these appendages has been 

 eliminated. The first persisting postoral segment (fig. 2 C, II) is that 

 of the mandibles (Md). 



If the cephalic lobe of the embryo represents the primitive arthro- 

 pod head, or at least an early stage in the head evolution, it was merely 

 a sensory outpost at the anterior end of the animal. At this period 

 the wormlike lobopod progenitors of the arthropods and the ony- 

 chophorans probably had no specific feeding organs outside the mouth. 

 Some of the legs behind the mouth evidently served for grasping 



