THE INSECT HEAD SNODGRASS 



445 



forms, and wingless species that are undoubtedly descended from 

 winged ancestors. The Onychophora {Peripatus and related genera) 

 are sometimes classed with the Arthropoda, but these curious, many- 

 legged animals are more evidently related to the annelid worms. 

 The latter include the well-known earthworms and many worms that 

 live in the ocean, all comprised in the Annelida, which also we shall 

 have occasion to mention frequently in the following discussions. 



I. GENERAL STRUCTURE OF THE HEAD AND FEEDING ORGANS 



The head of an animal may be defined as the specialized anterior 

 part of the body. The organs of feeding are, primarily, the mouth 

 and whatever part of the alimentary canal serves for the ingestion 

 of food; and secondarily, external structures functionally asso- 

 ciated with the mouth. In insects, as we have seen, the head is a 

 consolidation of several anterior body segments; the mouth is an 



Pst 



Mth- 



A 



MtK 



Figure 1. — Head structures of annelid worms 



A, earthworm. B, marine worm, Nereis virens. Mth, moutli ; Pip, palpus ; Piid, 



parapodium ; Pst, prostomium ; Tl, tentacles. 



aperture on the ventral wall of the head; the ingestive part of the 

 alimentary canal is the stomodeum; the external organs of feeding 

 are appendages and lobes of the head associated with the mouth, 

 known collectively as the mouth parts. 



C ephalization. — A head is a prominent feature in the organization 

 of nearly all animals ; but all animals do not have a head to the same 

 degree, which is to say, cephalization has not progressed equally 

 far or accomplished the same results in all cases. The term cephali- 

 zation (from Greek KtcftaXrj^, a head) means the structural specializa- 

 tion of the anterior end of the body for whatever purposes the 

 animal has found it advantageous to have its anterior end physio- 

 logically specialized. Anatomical cephalization assumes that in 

 the evolution of any group of animals there was a time when the 

 ancestors of the group did not have a head or that the primitive 

 head was nothing more than the pole of the body directed forward 

 during progression. The habit of moving always with the same 



