NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS IO7 



EVOLUTION OF THE HEAD 



The prostomial acron does not constitute the definitive head of any 

 known arthropod ; there is always added to the acron at least one 

 postoral somite, and generally the definitive head includes from four 

 to six somites. A head composed of the acron and one somite, how- 

 ever, recurs so frequently, either in the adult stage or in ontogenetic 

 development, as to suggest that a simple head structure of this kind 

 (%• 39 Q Prtc) represents the earliest stage in the evolution of the 

 more complex types of arthropod head. It may hence be termed the 

 protocephalon. The best example of a functional protocephalon is to 

 be seen in the anostracan Branchiopoda (fig. 50 A), in which the 

 definitive head is a large cephalic lobe {Prtc) bearing the eyes, both 

 pairs of antennae, and the labrum. The protocephalon is unquestion- 

 ably the primitive head of all the mandibulate arthropods. There is 

 no direct evidence, however, that it ever occurred as a specific stage 

 in the evolution of the Trilobita or the Chelicerata, and hence, in the 

 ancestors of these groups, and in the protarthropods generally, the 

 primitive head may have been merely the prostomial acron. 



Crampton (1928) applies the term "archicephalon" to a supposed 

 stage in the cephalic evolution of the arthropods when the head con- 

 sisted of the procephalic region and the mandibular somite. That 

 such a stage occurred relatively late in the phylogenetic history of 

 the head, however, is clearly shown in the ontogeny of the Mandibu- 

 lata, in which the primitive embryonic head is always a cephalic lobe 

 bearing the first antennae and usually including the second antennal 

 somite, while the gnathal somites are still a part of the body region. 

 Antedating this protocephalic stage, however, there must theoretically 

 have been a truly primitive stage when there was no head structure 

 other than the prostomium. The prostomium, therefore, which be- 

 comes the acronal region of the definitive head, is the only stage in 

 the evolution of the arthropod head that might properly be termed 

 the "archicephalon." 



The trilobite branch of the protarthropods is characterized by a 

 lateral expansion of the body, produced by an extension of the lateral 

 margins of the tergal plates into long flat lobes (fig. 36 E, 48 D) . The 

 dorsal surface of the body thus presents a median elevated area 

 (rhachis) accommodating the alimentary canal, and broad depressed 

 lateral areas (pleurae). On the under surface the true venter (fig. 

 48 D, V) is the area between the leg bases, the areas laterad of the 

 legs being the ventral doublure {dhl) of the dorsum. The appen- 

 dages bear long coxal epipodites (Eppd) supporting branchial 

 lamellae or filaments. 



