90 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



On the assumption that the Arthropoda and the Onychophora 

 are derived from generalized annehds, the primary head of the 

 onychophoran-arthropod ancestors must have been the prostomium. 

 The prostomium, therefore, constitutes the archicephalon in the series 

 of articulate animals. In the polychaete annelids the prostomium 

 (%• 39 A, Prst) supports two pairs of sensory appendages, the 

 tentacles {Tl) and the palpi {Pip), and often a median anterior 

 tentacle, and bears dorsally the eyes and the nuchal organs, while 

 between it and the first somite (/) is situated ventrally the mouth 

 (Mth). The neural elements of the prostomium, probably including 

 originally a median apical ganglion and several paired ganglia devel- 

 oped in connection with the sensory organs (fig. 9 B), unite to form 

 the composite suprastomodaeal nerve mass known as the brain, or 

 archicerebrum (C, D, Br). 



The young arthropod embryo characteristically has at the anterior 

 end of the body a large cephalic lobe (fig. 39 B, Acr). On this head 

 lobe are developed the eyes, both simple and compound (£), the 

 first antennae (lAnt), in some cases a pair of transient preantennal 

 rudiments (Prnt), and the labrum (Lm). The neural elements of 

 the embryonic head, which may include an anterior median ganglionic 

 rudiment and as many as four paired lateral rudiments, soon unite 

 to form the suprastomodaeal brain. The exact parallelism in structure 

 and development between the cephalic lobe of the arthropod embryo 

 and the prostomium of the polychaete worm (A) certainly suggests 

 a morphological identity between the two organs. In neither is there 

 ever any external mark of segmentation, or direct evidence of the 

 confluence of more primitive segments. 



SoUaud (1923, 1933), from his study of the development of the 

 crustacean Leander, contends that the embryonic head region (fig. 

 38 D, E, Prst) on which are developed the procephalic (ocular) lobes 

 (Pre) and the first antennae {lAnt) must represent the annelid 

 prostomium, since the first intersegmental groove runs behind the 

 first antennae, and there is no external evidence of segmentation 

 before it. Moreover, in the procephalic nerve ganglia, he says, only 

 a slight constriction occurs at an early stage between the ocular, or 

 protocerebral, parts and the antennal, or deutocerebral, parts. SoUaud 

 asserts, therefore, that there is no valid reason for the commonly 

 accepted view that the first antennae are homodynamous with the 

 following appendages in the sense that they are the appendages of a 

 primarily postoral somite that has been secondarily incorporated with 

 the prostomium. The first antennae of Leander, he shows, remain 

 uniramous, while almost from the beginning the second antennae 



