106 P. W. GAMBLE, M.SC, AND J. H. ASHWORTH, B.SC. 



the brain. The posterior cornua seem to be specially connected with 

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

 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. X, Figs. 48 and 49) a similar condition in 

 Clymene. It is only at this point that the nervous system of the adult 

 Arenicola marina can be said to have an epidermal position. Else- 

 where it is separated from the epidermis by the circular musculature. 



The circiun-oesophageal nerve-connectives arise^from the large anterior 

 cornua in the form of two thick coixls, covered on their outer surfaces 

 by ganglion-cells (Figs. 20, 21, 25, Oe. Comm.). From them a pair of 

 short nerves (Fig. 26, OT. N.) arise supplying the otocysts, and several 

 longer ones are distributed to the oral papilla 3 , of the ventral region of the 

 mouth. The line of the connectives is marked externally by the "metas- 

 tomial groove" (PL IX. Fig. 19, C), and the commencement of the 

 ventral cord by the junction of these grooves which occurs on the 

 ventral surface just in front of- the first chsetigerous annulus. The 

 nerve-cord is protected by a delicate connective-tissue sheath, a thin 

 sheath of circular muscle, and a thin layer of epidermis. Though 

 nearly circular in section it is somewhat flattened from above downwards, 

 but exhibits scarcely a trace of segmentation externally or internally. 

 The ganglion-cells are arranged in two ventral groups, while the fibrous 

 portion of the cord is dorsal. In the tail the ganglionic masses increase 

 in size, and are separated from the skin by a thicker layer of circular 

 muscle-fibres. Two "giant-fibres" are present in the branchial region, 

 a single one only in the anterior and tail region. 



From the chord a paired series of nerves is given off with great 

 regularity, one opposite each groove separating the annuli of the 

 somites, so that there are five nerves on each side of the body in each 

 somite. These lie in the body-wall just beneath the circular layer of 

 muscle, and, in some places where this layer becomes obsolete, they lie 

 just under the epidermis. Dorsally these nerves thin out and become 

 very difficult to trace. 



Sense-organs. — There is no doubt that the prostomial lobes, the 

 nuchal organ, and the otocysts are sense-organs ; but there are, in 



