NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 43 



the Hesionidae (fig. i8D), the Aphroditidae (B), the Nereidae 

 (E, F) and other errant famihes, that the roots of the palpal nerves 

 are closely associated in glomerulous bodies with the stalks of the 

 corpora pedunculata, which fact, Hanstrom points out, clearly sug- 

 gests that the corpora pedunculata had their inception as association 

 centers for the sensory nerves of the palpi. Much importance attaches 

 to a study of the corpora pedunculata in connection with annelid and 

 arthropod phylogeny, because bodies very similar in position, struc- 

 ture, and variations are characteristic features also of the brain of 

 Onychophora and Arthropoda. 



The relative positions of the principal internal structures of the 

 polychaete brain, it should be noted for later comparison with the 

 onychophoran and arthropod brain, are as follows : Anteriorly and 

 dorsally are the corpora pedunculata (fig. i8 B, D, E) ; closely 

 associated with the stalks of the latter are the palpal glomeruli (A, B, 

 F, PlpGlin), and the glomeruli are connected by a palpal commissure; 

 behind the corpora pedunculata is the optic commissure (D, E, 

 OpCom) ; and in the posterior part of the brain are the nerve centers 

 of the nuchal organs and a nuchal commissure (E, NConi). The 

 stomodaeal connectives attach to the ventral surface of the brain. 



The number of nerves given off from the brain is highly variable 

 according to the development of prostomial sense organs. In Poly- 

 gordius (fig. i8 A) there is but a single pair of cerebral nerves, 

 which innervate the tentaclelike palpi, while in such forms as Nereis 

 (fig. 17 A) an elaborate innervation of the prostomial walls, the sense 

 organs, and the stomodaeum proceeds from the brain. 



The principal stomodaeal nerves of the Polychaeta arise in some 

 families from the first ganglia of the ventral nerve cords, or from the 

 brain connectives near these ganglia, while in others they come from 

 the upper parts of the connectives or from the back of the brain. 

 It is contended by Hanstrom (1927, 1928), therefore, that in the 

 second case the primitive first ganglia of the cords have been drawn 

 forward and united with the brain, forming thus in certain polychaete 

 families a posterior part of the definitive brain corresponding with 

 the tritocerebral lobes of the arthropod brain, which always have 

 connections at least with the stomodaeal (stomatogastric) system 

 of nerves. 



The stomodaeal innervation of the Polychaeta is most elaborate in 

 those forms that have a large and eversible stomodaeal proboscis, and 

 in such cases the innervation of the organ may be derived from so 

 many sources that the evidence adduced in favor of Hanstrom's theory 

 is not convincing. In Nereis, for example, Hamaker (1898) describes 



