48 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I42 



centers are more differentiated. It is argued, therefore, by the above- 

 named authors that the oculo-antennal part of the arthropod brain 

 represents the prostomial brain, or archicerebrum, of the polychaetes, 

 and that the procephaHc part of the adult head is derived from the 

 annelid prostomium. Secondarily added to the brain in the insects 

 and myriapods are the ganglia of the first postoral body segment, 

 which become the tritocerebral brain lobes. The commissures of the 

 optic and antennal centers are intracerebral and suprastomodaeal. The 

 commissure of the tritocerebral ganglia is free beneath the stomo- 

 daeum. 



This concept that the adult head consists of a primitive cephalic 

 lobe equivalent to the prostomium of the annelids and four second- 

 arily added postoral somites has been maintained in a recent study 

 by Butt (i960) on the embryonic development of the arthropod head. 

 The prostomial part of the head is represented in the embryo by the 

 blastocephalon, w^ithin which are differentiated from the archicere- 

 brum the ocular and antennal centers of the definitive brain. 



The principal objection that has been urged against this interpre- 

 tation is based on the occurrence of paired cavities in the mesoderm 

 of the embryonic cephalic lobe. The presence of mesodermal cavi- 

 ties, regarded as coelomic sacs, has been recorded in the labrum, in 

 the preantennal region, and associated with the first antennae. Most 

 writers discount the significance, or even the verity, of the labral 

 cavities, but the preantennal and antennal sacs are taken as evidence 

 of segmentation. The preoral mesoderm has been shown in Onychoph- 

 ora and Arthropoda to be formed by forward growth of postoral 

 mesoderms, the labral mesoderm being derived from the preantennal 

 mesoderm. This fact cannot mean necessarily that the forward-grow- 

 ing mesoderm represents anteriorly migrating segments, and it throws 

 some doubt on the segmental value of the transient cavities that sub- 

 sequently appear in it. 



The mere presence of paired cavities in the trunk mesoderm is 

 accepted by some zoologists, especially embryologists, as unques- 

 tioned evidence of body segmentation. If, then, any pair of cavities 

 in the mesoderm, particularly when associated with nerve ganglia, 

 defines a segment, there is no further argment on the subject. How- 

 ever, in the adult animal a segment is a motor unit of the body with 

 an intrasegmental somatic musculature. In this sense, therefore, the 

 contention that the blastocephalon is a segmented region implies the 

 assumption that at some time in the history of the insect it consisted 

 of individually movable rings. Clearly this assumption is purely 



