I06 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



ganglion, paired protocerebral and optic ganglia, paired preantennal 

 ganglia, and paired first antennal ganglia. That these ganglia belong 

 to the preoral acron (Acr) is shown by the fact that the paired 

 ganglia are always connected by preoral commissures. The cephalic 

 mesoderm extends forward from the somatic mesoderm bands, and, 

 in its fullest development, surrounds the mouth anteriorly ; it may 

 become excavated by cavities corresponding with the first antennae 

 (j), the preantennae (<?), and the labrum (/). The development of 

 the prostomial nerve ganglia and mesodermal cavities is determined 

 probably in all cases by external structures (appendages or sense 

 organs), but the acronal neuromeres and coelomic sacs, because of 

 their radial position around the mouth, cannot have the same relation 

 in the body structure as have their postoral counterparts that are 

 .transversely opposed to each other. For a like reason there is no 

 prostomial metamerism of the muscular system. The same concept 

 may be applied to the preoral lobe of the polychaete annelids (A), 

 assuming that potentially the annelid prostomium might have a full 

 quota of coelomic sacs corresponding with its appendages, which actu- 

 ally it does not have. The tritocerebral somite of the arthropods thus 

 represents the first postoral somite of the annelids. The tritocerebral 

 ganglia are secondarily united with the preoral cerebrum in the Ony- 

 chophora and in most of the Arthropoda, and always have a postoral 

 commissure; the corresponding appendages are the jaws of the Ony- 

 chophora, the chelicerae of the Chelicerata, and the second antennae 

 of the Mandibulata. 



The definitive arthropod brain more closely resembles the brain of 

 the Polychaeta than that of the Onychophora. Its principal part is 

 the protocerebrum, formed of a median apical ganglion and the first 

 pair of lateral ganglia, with which are connected the optic ganglia. 

 The preantennal ganglia lose their individuality in the general cerebral 

 mass. The antennal ganglia form the deutocerebral lobes, but the 

 latter take a forward position beneath the protocerebrum, with the 

 result that, in the definitive condition, the antennal nerves arise 

 anteriorly belozv the optic lobes. In the Onychophora, on the other 

 hand, though the antennae are anterior, the brain maintains a hori- 

 zontal position (fig. 25 A, C) with the antennal commissure behind 

 the optic region, and the antennal tracts (AntT) traverse the dorsal 

 part of the brain above the optic lobes. The tritocerebral ganglia are 

 united with the primary cerebrum in the Onychophora and in nearly 

 all the Arthropoda, but the union would seem to have taken place 

 separately in the two groups, since in some of the lower Crustacea 

 the corresponding centers are independent ganglia on the nerve cords, 

 as they are in most of the Annelida. 



