22 F. W. GAMBLE AND J. H. ASHWORTH. 



very short distance, a second connective-tissue partition divid- 

 ing the smaller posterior cornua (P. Cr.), which gradually taper 

 oflF and end at the hinder edge of the nuchal organ (PL 5, 

 fig. 25). 



Sections of the prostomium of the littoral variety of 

 Arenicola (immature specimens, 4" long) exhibit a thick 

 covering of ganglion- and glia-cells, forming the dorsal surface 

 of the brain (fig. 24) ; a central fibrous portion ; and a strong 

 ventral membrane, into which the greater part of the pro- 

 stomial muscles are inserted, though a few fibres are attached 

 in front of and between the anterior cornua (PI. 5, fig. 25). 

 In older specimens, and particularly in mature examples of the 

 " Laminarian^' variety, the ganglion-cells are more scattered, 

 and in other ways the brain shows greater differentiation. The 

 anterior cornua, for example, are not only deep and thick, but 

 give oflT from their dorsal surface short stout branches, along 

 which the ganglion-cells are scattered, and which supply the 

 prostomium. The central fibrous part of the brain also grows 

 out ventrally in these large examples, separating the hitherto 

 compact layer of cells and carrying them outwards or leaving 

 them in clumps, and not evenly arranged as in young Areni- 

 cola. 



From the anterior cornua a large nerve arises on each side, 

 in front of the origin of the oesophageal connectives. It passes 

 out to the under surface of the epidermis, and supplies the 

 papillae on the upper surface and the sides of the mouth. The 

 epidermis of the prostomium itself is in close contact through- 

 out its whole length with the ganglionic covering of the pro- 

 cesses arising from the dorsal and lateral surfaces of the brain. 

 The posterior cornua seem to be specially connected with the 

 nuchal organ, against which they lie and terminate (PI. 4, 

 fig. 21). 



The most remarkable histological feature of the brain is the 

 close contact between the large ganglion-cells of its upper 

 surface and the sensory epithelium of the prostomium (figs. 20 

 and 24). Racovitza (1896) has figured (PI. 5, figs. 48 and 49) 

 a similar condition in Clvmene. It is only at this point 



