ON THE ANATOMY OF H[STRTOBDELLA HOMART. 321 



7. The Nervous System (figs. 15, 21, and 28). 



'J'he nervous system extends throughout the body, and is 

 composed of a brain, oesophageal commissures, and ventral 

 nerve-cord, with ganglia at intervals corresponding to the 

 exteimal segmentation. The brain is situated well forward 

 in the head, its main mass being anterior to the oral opening, 

 and close to the dorsal surface. It is composed externally 

 of a mass of nerve-cells surrounding a clear fibrous core. 

 The uervo-cells are distributed over its dorsal surface. 

 Behind, the brain is deeply cleft ventrally, descending in 

 lateral lobes on either side of the anterior part of the mouth. 

 This cleft runs forward, forming a small closed sinus in the 

 anterior end of the organ. 



The brain terminates rather abruptly at a point about on 

 a line with the anterior third of the jaws; here it gives off 

 two fine commissures that run directly ventralwards and 

 backwards, connecting it with the first ganglion of the ventral 

 nerve-cord. At the point where these come off some fibres 

 go to the anterior legs, and others run directly backwards in 

 the dorsal region. They probably correspond with the 

 " nerfs sympathiques " of Foettinger. In addition to these, 

 the brain supplies nerves to the anterior tentacles. 



The commissures are closely applied to the oesophagus, and 

 are difficult to follow in sections on account of their small 

 size. 



At about on a line with the posterior boundary of the 

 brain, and slightly in front of the anterior feet, there is a 

 small ciliated pit on either side of the head. The anterior 

 lip of this protrudes slightly, forming a sort of papilla. This 

 pit is undoubtedly sensory in nature, and appears to have 

 some fine nerve-fibres running to it from the brain. The 

 nerve-cells of the dorsal surface of the brain are distinctly 

 differentiated from the cells of the ectoderm. They are 

 recognisable by the elliptical outline of their nuclei, and the 

 marked way in which they take the stain when treated with 

 the heematoxylin mixtures. As compared with the ectoderm 



