NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 45 



five pairs of stomodaeal nerves, two pairs of which proceed from the 

 anterior part of the brain (fig. 17 A, a, c), a third pair (/) from 

 small ganglia on the upper ends of the stomodaeal connectives, a 

 fourth (m) from the ganglia of the first peristomial cirri, and a 

 fifth (p) from the suboesophageal ganglion. In the earthworm, 

 Lumbricus, the stomodaeal innervation arises from the connectives 

 between the brain and the first ganglia of the cords (fig. 17 C, StGng). 

 Other examples would only show further inconsistencies iri the origin 

 of the nerves that supply the annelid stomodaeum. We can, therefore, 

 most readily agree with Gustafson (1930), who concludes that no 

 homology exists between the stomodaeal nervous system of the Anne- 

 lida and that of the Arthropoda. Gustafson points out, furthermore, 

 in reference to Hanstrom's theory of transposed ganglia, that there 

 is no concrete evidence of the transfer of a pair of ventral ganglia to 

 the brain in any of the annelids, whereas in the arthropods there is 

 conclusive proof that the tritocerebral ganglia have been secondarily 

 united with the brain. In the higher arthropods, moreover, the ganglia 

 of the stomodaeal nervous system are derived directly from the 

 ectodermal wall of the stomodaeum itself, and their definitive nerve 

 connections with the central system appear to be secondary. 



THE EYES 



Light-receptive organs in the form of eyes are widely present in 

 the Polychaeta. The polychaete type of eye is fundamentally a 

 vesicular ingrowth of the integument (fig. 28 C, D, E), the retinal 

 cells being epithelial cells of the vesicle wall converted into primary 

 sense cells by the extension of their inner ends as nerve fibers. In the 

 simpler forms of eyes the cuticula may form a mere plug in the cavity 

 of the retinal sac (fig. 19 D), but usually the ingrown part of the 

 cuticula is enlarged and becomes a lenslike body, either connected with 

 the surface by a cuticular strand, or entirely shut in by the union of 

 the lips of the retinal sac (G, Ln). The outer ends of the retinal 

 cells form optic rods, converging upon the inner surface of the lens 

 (E, G, c), which contain the distal parts of the neural fibrillae (E, 

 w/&/), but the apposed surfaces of adjacent rods do not form rhab- 

 doms (F), as they do in the Arthropoda. 



THE NEPHRIDIA AND THE GENITAL DUCTS 



The most primitive excretory organs of the annelids are the proto- 

 nephridia of trochophore larvae. These are minute tubes, one or two 

 pairs, extending from the body wall into the haemocoele, where they 



