NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 89 



are separately derived from an ancestral form represented by the 

 polychaete trochophore, since the adult arthropods have too many 

 features in common with adult annelids that are not yet present in 

 the trochophore. The common ancestor of the two groups, therefore, 

 is to be found in a much later stage of annelid development than that 

 of the trochophore. The trochophore and the nauplius are specialized 

 larval forms, adapted in their general shape and structure to a tem- 

 porary pelagic life; but, since they represent an early stage of phylo- 

 genetic development, and probably originated as larvae at an early 

 phylogenetic period of evolution in their respective groups, they 

 necessarily show primitive characters in their basic organization. 



THE CEPHALIC SEGMENTATION AND THE DEVELOPMENT OF THE BRAIN 



The question of the number of segments that enters into the com- 

 position of the arthropod "head" has been widely investigated and 

 discussed, but with such lack of uniformity in the results as to lead 

 to the suspicion that interpretation of the observed facts has been too 

 much influenced by theoretical considerations. The writer believes 

 that a more literal acceptance of the known facts of embryonic 

 development in the case of the arthropod head will give a simpler 

 and more satisfactory concept of the fundamental cephalic structure 

 than that which has been current for several decades. 



In the first place, it should be understood that there is no specific 

 "arthropod head." The cephalic structure is a variable combination 

 of segments, and the number of cephalized segments may be quite 

 different in different arthropod groups, or even within a single major 

 group. The more complex types of head, such as occur in the Man- 

 dibulata, include an anterior procephalic region bearing the labrum, 

 the eyes, and two pairs of antennae, and a posterior gnathal region 

 bearing the mandibles, the first and second maxillae, and in some 

 forms the first maxillipeds or also the second maxillipeds. In the 

 Trilobita the so-called "head" is a combination of at least four postoral 

 somites with the prostomial acron, and the "prosoma" of the Chelic- 

 erata is a similar composite structure, except that it contains six 'or 

 eight somites. On the other hand, in many of the Crustacea, the true 

 head is a primitive structure corresponding with the procephalic part 

 of the head in other mandibulate groups. However, differences of 

 opinion as to the number of somites involved in the head composition 

 pertain chiefly to the procephalic region, since the segments of the 

 gnathal region are usually distinct in the embryo, and are readily 

 identified by their appendages. 



