W. R. Coe — Anatomy of Cerebratulus lacteus. 499 



and the former begin. As an arbitrary point, however, I would sug- 

 gest that the origin of the vagus nerves be taken as the posterior 

 extremity of the ventral lobes. The vagus nerves, as described 

 below, pass to the lateral borders of the mouth and oesophagus. 

 The lateral cords pass backward in the body-wall (Plate X, fig. 4), in 

 a median horizontal plane, to the posterior end of the body without 

 appreciable change except as to size. Throughout their length they 

 are connected, both above and below, by a loose plexus of nervous 

 tissue (Plate XI, fig. 1, np) which forms a complete cylinder just 

 external to the circular muscular layer (cm). Most of the fibres 

 which go to innervate the integument, body-muscles and other tissues 

 first pass from the lateral cords into this plexus and thence to their 

 destination. From this plexus multitudes of nerve-fibres pass 

 peripherally, between the muscle-bundles, to the integument. Like- 

 wise at short intervals fibres are given off to the digestive canal. 

 Such fibres have to pass through the circular and inner longitudinal 

 muscular layers. 



In the median line the plexus is thickened to form a more or less 

 distinct dorsal, or medullary, nerve (Plate X, fig. 4, dn). Although 

 this nerve is much less conspicuous in this than in many related 

 species, it can be traced without difficulty to the dorsal commissure 

 of the brain. 



A pair of large nerves, the vagus (Plate X, figs. 4, 6, 8, vn / Plate 

 XV, fig. 8), arise from the internal borders of the ventral brain - 

 lobes near the origin of the lateral cords. They are connected with 

 a strong commissure (cv) and pass downwards and outwards along 

 the edges of the mouth, to supply this organ and the anterior portion 

 of the oesophagus. The commissure is fully as large as either vagus 

 nerve and is provided with ganglion-cells on its dorsal border. 



The proboscis is supplied with a pair of large nerves (Plate X, 

 figs. 6, 8, pn) from the anterior border of the strong commissure of 

 the ventral brain-lobes. From their origin these nerves run anteri- 

 orly near the proboscis-sheath and enter the proboscis at its attach- 

 ment to the cephalic muscles. After reaching the proboscis they 

 appear crescent-shaped in transverse section and lie in the midst of 

 the muscles on each side (Plate XII, fig. \\,pn). Their position in 

 the proboscis is thus similar to that of the lateral cords in the body- 

 wall. The ends of the crescents elongate until they meet above and 

 below to form a nervous plexus (fig. 1 2, np) between the circular 

 and inner longitudinal muscular layers. 



Histology. — The nervous tissues of which the brain and lateral 



