HABITS AND STRUCTURE OP ARENICOLA MARINA. 23 



that the nervous system of the adult Arenicola marina can 

 be said to have an epidermal position. Elsewhere it is separated 

 from the epidermis by the circular musculature. 



The circum-oesophageal nerve-connectives arise from the large 

 anterior cornuain the form of two thick cords, 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 supply- 

 ing the otocysts, and several longer ones are distributed to the 

 oral papillae of the ventral region of the mouth. The line of 

 the connectives is marked externally by the " metastomial 

 groove^' (PI. 4, fig. 19, C), and the commencement of the ven- 

 tral cord by the junction of these grooves, which occurs on the 

 ventral surface just in front of the first chaetigerous 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 in- 

 crease 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 cord 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 addition, certain other structures, such as the 

 setse^ of the notopodia and some of the buccal papillae, which, 



1 Retzius has described free nerve-endings on these setae. ' Biologiska 

 Foreningens Forhandliugar,' Bd. ill, Hefte 4 — 6, 1891, p. 85. 



