31U 



COMPARATIVE ANATOMY. 



of masses of ganglia, which lie uear the oesophagus (Fig. 165-, d). 

 A larger gaugliou lies transversely across and below the oesophagus, 

 or rather, owing to the downward bend of the a3sophagus, behind it 

 (in the Terebratulida) . Two nerve-trunks pass off from it towards the 

 posterior region, and break up into nerves for the stalk ; they are pro- 

 vided with swellings {ii'). The nerves for the ventral lamella of the 



mantle are given off from 

 the swellings on these 

 trunks. From the large 

 ganglion, however, a nerve- 

 trunk is given off, on each 

 side, to the dorsal lamella 

 of the mantle, as well as a 

 nerve to the arms. Two 

 fine filaments surround the 

 oesophagus and pass into a 

 small ganglion, placed in 

 front of, and therefore on 

 the dorsal side of, the oeso- 

 phagus; this is connected 

 with the other ganglion by a 

 commissure. In this way an 

 oesophageal ring is formed, 

 and the only question is 

 whether the small upper 

 ganglia represent central 

 ganglia or not. If so, then 

 there is this peculiarity, that 

 the nerves for the arms 

 arise from the ventral gan- 

 glia, and we can hardly re- 

 gard the arms themselves as 

 homologous with the ten- 

 tacles of Vermes, even if it 

 could be shown tliat the gan- 

 glionic parts in the oesopha- 

 geal ring had altered their 

 position. The ventral gan- 

 glionic mass must appa- 

 rently be compared with a 

 shortened ventral gangli- 

 onic chain ; but to make a safe comparison we must have more exact 

 information as to the facts. 



The slight development of the superior ganglia is correlated 

 with the absence of higher sensory organs, and this absence 

 appears to be an acquired condition, for the four pigment spots, 

 found on the first segment in the larva, point to the existence 

 of visual organs (Fig. 163), and lead us to suppose that there were 

 eyes in the ancestral forms. The two vesicles found in another larval 

 form similarly point to the previous existence of auditory organs. 



Fig. 165. Nervous system of Waldheimia, 

 from the dorsal smface. The dorsal valve has 

 been removed, as well as the left half of the 

 dorsal mantle, D. V Left half of the ventral 

 lamella of the mantle. P Stalk, d OEsophagus, 

 cut through. (A pair of ganglia, Trhich lie in 

 front of the oesophagus, and which are connected 

 with the ganglion {n) by fine filaments, are 

 not figured.) n Anterior, n ' Posterior oesopha- 

 geal ganglion, f/c/ Generative organs. liiOcclusar- 

 muscle, m' Divaricator. -iii" Ventral adjuster. 

 m'" Accessory Divaricator (after A. Hancock). 



