ORIGIN OF THE ARRANGEMENT OF CEPHALIC FUNCTIONS. 9 



number of like metameres, each one complete in itself, that is, having all the 

 organs of an ideal metamere. This assumption is untenable. A considerable 

 number of cephalic, or anterior metameres, even approximately complete or 

 perfect, rarely, if ever, occur in any animal outside those pictured in text-book 

 diagrams. It is certain that no such condition occurs in the arachnids. While 

 it may be assumed that metameric growth tends to produce a linear series of like 

 parts, it is clear that it does not do so in reality. The first products of apical 

 growth must necessarily differ from the last, because different conditions are cre- 

 ated by apical growth at each successive stage of its progress. The actual result, 

 therefore, is a linear sequence of unlike structures and functions for a given number 

 or generation of metameres. This particular sequence becomes unbalanced and 

 remodelled with the appearance of the next generation. But on the whole a 

 definite linear succession of unlike organs becomes established at a very early 

 period in the evolution of segmented animals; and it follows a logical, inherently 

 necessary order, that is never completely lost or disguised. 



With the elongation and increase in size of the primitive trunk the ingestive, 

 gustatory, locomotor, cardiac, and respiratory functions become more localized, 

 their position being determined, in part, by the necessary conditions for their 

 activities, and in part by the historic order in which they became established; for 

 the location of any new function is limited to the territory that is not already pre- 

 empted by other organs. For that reason we find that the most essential organs 

 are the first to develop, and they arise from the oldest parts of the body, that is, 

 from the more anterior and median neural surface; the" organs of more recent 

 origin arise on the haemal and caudal sides of the older ones. 



The primary sense organs, i.e., the parietal and lateral eyes, the olfactory 

 organs, and the coordinating centers (forebrain) are already definitely located in the 

 procephalon of the nauplius, which probably represents, in part, the remnants 

 of a trochosphere. These organs are, therefore, of very great antiquity. They 

 retain their original position throughout the entire range of the arthropod-verte- 

 brate phylum, and by the root-like extension of their nerve fibers establish re- 

 lations with the new metameres as fast as they are formed. 



Hence the primary sense organs and the primary coordinating centers are 

 located at the anterior end of the body, not, as is frequently asserted, because the 

 body moves head first, or because of any necessary correlation between the location 

 of the brain and sense organs (Parker), but because the head is the oldest part of 

 the animal, and because these particular sense organs and nerve centers were, in 

 a historic sense, the first ones to be definitely established, taking their origin back 

 to a period when the primitive head was the whole body. 



With the appearance of the first postcephalic metameres, arose the first 

 gustatory organs, and the first swimming, grasping, crushing, and chewing ap- 

 pendages. They were necessarily located immediately behind the primitive 

 head, in the oral region. With the addition of another generation of metameres, 

 the body became heavier and larger, and the appendages on the new metameres 



