The nervous system and subradular organ in two genera of Solenogastres. 401 



The phaiynx is of medium length and passes without any special 

 modification into the stomach-intestine. On the ventral pharyngeal 

 wall the well developed polystichous radula is placed, its relations 

 being essentially the same as in the Glossophora generally. Immediately 

 beneath its forward border are two patches of high columnar cells 

 (Fig. 5 s. r. o), each group being capable of retraction within a sheath 

 or being everted and fully exposed. Both are innervated by nerves 

 from ganglia not hitherto described and I shall attempt to show that 

 they are probably to be considered as the homologue of the sub- 

 radular organ of the Chitons and some of the Prosobranchs. 



In Rhopalomenia the mouth is of about the same size as in 

 Proneonienia and the sense organs with which it is supplied are like- 

 wise similar and of the same general arrangement. Differences occur 

 in the opening into the pharynx, which is relatively very narrow, and 

 in the slender greatly elongated character of the pharyngeal tube. At 

 a distance from the front end of the body of upwards of twice the 

 diameter of the animal the pharynx unites with the stomach-intestine 

 which continues forward as a blind sac to a point somewhat in front 

 of the brain. Close to the junction of the pharynx and gut the sali- 

 vary glands are attached and at this point the buccal and subradular 

 ganglia are situated. The ladula is absent and a well defined sub- 

 radular organ is also lacking. Immediately above the subradular 

 ganglia are high columnar cells that may perhaps be considered the 

 rudimentary or diffuse sense organ. 



In the present species of Proneomenia the brain is of medium 

 size (Figs. 1, 4) and is situated in front of the unpaired salivary 

 gland in a depression that probably marks the dividing line between 

 the mouth and pharynx. From its anterior and lateral borders three 

 pairs of nerves (Fig. 1 n) arise and develop branches some of which 

 may be traced to the body wall and probably to the hypodermal sense 

 organs anterior to the hinder border of the mouth. The remaining 

 fibres remain attached to the digestive tube and unite with a sheet 

 of ganglion cells that are almost coextensive with the buccal wall. 

 In favorable sections these ganglionic masses appear to be arranged 

 in small groups each of which is continuous with two or more nerve 

 bundles, thus forming a close-meshed plexus. From this network some 

 fibres pass outward and appear to unite with somatic muscles and 

 hypodermal sense organs while others may be traced far into the 

 ciliated buccal ridges mentioned previously. Other nerves probably 

 from the same source pass into the cavity of each cirrus. These 



