2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I42 



Since this end comes first in contact with whatever may serve as 

 food, the mouth also is usually at or near the anterior end. Thus the 

 anterior part of the animal has become structurally a head bearing 

 the orienting sense organs and the mouth. All animals that have a 

 stomach have a mouth ; the mouth is as old as the blastopore. Yet it 

 would seem that the primitive animals had no jaws or other special 

 feeding organs associated with the mouth. In their later evolution 

 the common need for such organs has been met in various ways. 

 Among the coelenterates a circle of grasping tentacles was developed 

 around the mouth. The earthworm ingests mud sucked in by a muscu- 

 lar pharynx, but some of its polychaete relatives developed teeth or 

 jawlike organs in the pharynx, which became eversible as a proboscis. 

 In the ancestral vertebrates two pairs of preexisting gill arches were 

 converted into jaws, which are pennanently within the mouth. The 

 arthropods are unique in that their feeding organs have been fash- 

 ioned from a pair or several pairs of legs behind the mouth. The 

 forelimbs or fingers of some quadruped or biped vertebrates, of 

 course, are often used for grasping food and putting it into the mouth, 

 but they have never become modified for biting and chewing. The 

 primitive arthropods, however, had so many legs they could well spare 

 a few for purposes other than that of locomotion. 



The adult head of an insect is a composite structure in which the 

 segments of the feeding appendages have been intimately combined 

 with a primitive head that was principally sensory in function. The 

 insect head is thus superior in many ways to the head of any other 

 animal in the number of functional units it contains. It is a cranial 

 structure provided with sense organs of numerous kinds, and a feed- 

 ing apparatus capable of being modified for feeding in various ways 

 on different kinds of food. The sensory organs include simple and 

 compound eyes, and a pair of antennae that are delicately sensitive to 

 touch and odor, and, in some cases, to sound. The feeding organs in 

 their simplest form serve for grasping, biting, and chewing, but all 

 together they may be modified and combined in different ways to 

 form a complex apparatus for sucking, or for piercing and sucking. 

 The insects in general are thus enabled to diversify their diet and to 

 get their food from many different sources. By contrast, the vertebrate 

 animals, provided only with jaws for biting and chewing and a tongue 

 for licking and lapping, are practically limited to one way of feeding. 

 Moreover, to sample any substance for food the vertebrate must take 

 it into its mouth, where the gustatory organs are located. The taste 

 organs of insects, on the other hand, are outside the mouth, very 



