446 ANNUAL REPOET SMITHSONIAN INSTITUTION, 1931 



end forward, therefore, first established the distinction between a 

 head end and a tail end in the animal, and was the precursor of 

 cephalization. 



A good example of well-established physiological differentiation 

 between the two poles of the body, but with a minimum of anatomical 

 cephalization, is seen in the Annelida, or segmented worms (earth- 

 worms and their marine relatives). The "head" of these worms 

 consists only of a small apical lobe of the body, called the prostomium 

 (fig. 1, Pst)^ which is highly sensitive and gives rise from its 

 ectoderm to the first ganglion of the central nervous system, but 

 it does not bear feeding organs, and the mouth (Mth) is located 

 behind it. The annelids show also a good example of simple seg- 

 mentation of the body, since the entire length of the worm behind 

 the prostomium is divided by circular grooves into short body parts, 

 or somites, commonly termed " segments." The mouth (Mth) lies 

 in the ventral wall of the first segment, or between this segment 

 and the base of the prostomium. The earthworms lack segmental 

 appendages, but some of the marine annelids have a series of mov- 

 able flaps along each side of the body, called parapodia (fig. 1 B, 

 Ppd). 



The Arthropoda are segmented animals having much in the basic 

 plan of their organization that resembles that of the Annelida; but 

 in most respects they are far more highly evolved animals than are 

 the segmented worms, and their complex segmental appendages, as 

 we have seen, are characteristic features of their anatomy. In the 

 arthropods, cephalization has progressed so far that the head con- 

 sists not only of the prostomium but of a varying number of the 

 anterior segments of the body, all intimately combined in a composite 

 head structure. The head, moreover, retains most of the appendages 

 of its component segments, which are structurally modified for 

 various purposes, and its united nerve ganglia form specially 

 developed nerve centers. 



The degree of cephalization, however, varies much even within the 

 Arthropoda. A relatively simple head occurs in many of the Crusta- 

 cea, in which the head consists of not more than three primitive body 

 segments combined with the prostomium (figs. 2 A, 16 A, Pre). A 

 head of this type of structure bears the eyes {E), two pairs of an- 

 tennal appendages {lAnt, 2Ant), the mouth on its under surface, and 

 a median prostomial lobe (Zm) before the mouth. The jaws {Md) 

 and other appendicular organs that function in connection with feed- 

 ing are carried on the body segments immediately following the 

 head. In some other crustaceans, however, the segments of the feed- 

 ing organs also are added to the head, and in such cases the entire 

 head complex possibly contains as many as six or seven segments. 



