NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 4I 



to the same somite, and, furthermore, it sometimes contains the 

 gangHa of more than one somite. In the polychaete family Amphinom- 

 idae there is, in addition to the median nerve cords, a pair of lateral 

 cords extending posteriorly from the brain (fig. 17 B, PdNv), which 

 unite the series of podial ganglia (PdGng) lying at the bases of the 

 parapodia (see Storch, 1912, 1913, Gustafson, 1930). The tetra- 

 neurous structure is regarded by Storch as representing the more 

 primitive condition of the annelid nervous system, though Gustafson 

 contends that it is probably secondary. According to a theory pro- 

 posed by Jeener (1928) the lateral line system represents a primitive 

 series of neuromuscular sensory organs, from which there has been 

 preserved and developed in the Sedentaria the sensorial elements, in 

 the Errantia the ganglionic elements, and in the Oligochaeta the 

 muscular elements. 



The annelid brain in its simplest form probably consists of a 

 homogeneous mass of neurocytes aggregated upon a fibrous com- 

 missure continuous on each side with the stomodaeal connectives, and 

 through the latter with the ventral nerve cords (fig. 19 A, B, Br). 

 With higher development, however, specialized groups of cells appear 

 in the cortex, and specific tracts of fibers are individualized in the 

 neuropile. A very simple brain structure occurs in the archiannelid 

 Polygordius (fig. 18 A), in which, according to Hanstrom (1929), 

 a pair of glomerulous association centers {PlpGlm) receive the roots 

 of the palpal nerves and are connected by a palpal commissure 

 (PlpCom). The peripheral sense cells of the palpi form ganglionlike 

 masses (SCls) at the bases of the appendages. Two posterior lobes 

 of the brain (NL) are connected with the nuchal organs, but eyes 

 and anterior tentacles are absent in Polygordius. 



In the active polychaetes, in which cephalic tentacles, palpi, eyes, 

 and nuchal organs are well developed, the brain takes on a more 

 complex form and may acquire a high degree of differentiation in its 

 internal organization. Particularly conspicuous are the paired cellular 

 and fibrous masses known as corpora pedunculata. Each of these 

 bodies consists of a cap of small chromatic cells lying in the upper 

 anterior part of the cortex (fig. 18 B, E, Gb), and of a stalk, or 

 pedunculus (Ped), composed of the neurites of the cap cells, which 

 penetrate the central part of the brain. Within the stalks the terminals 

 of the neurites (B, d) form synaptic associations between fibers from 

 all other parts of the brain and from the ventral nerve cords (a, h, c). 

 A simple development of the corpora pedunculata is shown by Han- 

 strom (1927) to occur in the Hesionidae (fig. 18 D), in which the 

 caps consist each of a single globulus of cells, and the stalks are 



