498 W. R. Coe — Anatomy of Gerehratuliis lacteus. 



situated almost back to the intestinal region, or so far back that the 

 dorsal vessel has left the proboscis-sheath and the epithelium lining 

 the alimentary canal has assumed the granular appearance so charac- 

 teristic of the intestine. The onl}^ available specimen had two effer- 

 ent ducts on one side and but one on the other. 



The cephalic lacunae in Valencinia (Plate XV, fig. 14) are par- 

 tially divided into a number of longitudinal spaces which, in the 

 region of the brain, coalesce into two lacuna similar to those in 

 Gerebratulus. There is a large number of efferent nephridial ducts. 

 In one specimen the first pair of nephridia, with the efferent ducts, 

 %vas entirely separate from the six succeeding pairs. There was one 

 unpaired efferent duct. 



The JVervoics System. 



The central nervous system is situated, except in the head, between 

 the circular and outer longitudinal muscular layers of the body-wall, 

 and consists of (a) the brain, (b) the pair of lateral nerve-cords, (c) 

 the median dorsal nerve, and (d) a nerve-plexus uniting b and c. 

 Nerves to supply the oesophagus, the proboscis, the integument, 

 special sense organs, etc., constitute the peripheral nervous sj^stem. 



The brain (Plate X, figs. 7, 8) is made up of two large, ganglionic 

 masses situated in front of the mouth and immediately back of the 

 attachment of the proboscis (Plate XII, fig. 1). Each ganglion con- 

 sists of a dorsal and a ventral lobe, and is connected with its fellow^ 

 above and below the proboscis-sheath, so that this latter organ is, at 

 its anterior portion, surrounded by a ring of nervous matter. The 

 brain, with its commissures, and the lateral cords have a central 

 fibrous core and a thick outer sheath of ganglion-cells. The dorsal 

 lobes of the brain (Plate X, fig. 8, d) are rounded and lie immedi- 

 ately above the ventral lobes, with w^hich they are directly continu- 

 ous. Connecting the fibrous core of the dorsal lobe of one side 

 with that of the other is a comparatively thin dorsal cominissure {dc). 

 A portion of the fibrous core continues back to the side-organs (so) 

 which lie close behind the dorsal lobes. A multitude of small fibres 

 pass from these lobes in all directions to the tissues of the head, 

 including the rhynchodseum, cephalic slits, integument, etc. These 

 nerves are for the most part sensory in their function. 



The elongated ventral lobes (Plate X, figs. 2, 8, v) continue directly 

 backward into the lateral cords. There is a ventral commissure 

 (fig. 2, vc) much larger than the dorsal, connecting the ventral lobes. 



The lateral cords (Ic) are the direct posterior continuations of the 

 ventral lobes, so that it is impossible to decide where the latter end 



