the habits and structure of arenicola marina. 105 



8. Nervous System and Sense-organs. 



This system is composed of the brain, the oesophageal connectives, 

 the ventral nerve-cord, and the nerves arising from these. We have 

 not been able to demonstrate a visceral nervous system. 



The brain (PL X., Figs. 25, 26) is placed in the prostomium, of which 

 it forms the chief part, being only separated from the epidermis by 

 blood-vessels lying in extensions of the ccelom. It is a small elongated 

 structure, measuring "75 mm. in length in ordinary shore lugs, and 

 1 mm. in the large " Laminarian " variety. At its anterior end the 

 brain is divided into two stout cornna (A. Cr.), separated by a cleft 

 containing blood-vessels. About the middle of the brain the cornua 

 unite, but only for a very short distance, a second connective-tissue 

 partition dividing the smaller posterior cornua (P. Cr.), which 

 gradually taper off and end at the hinder edge of the nuchal organ 

 (PI. X, 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 prostomial muscles are inserted, though a few fibres are 

 attached in front of and between the anterior cornua (PI. X., 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 off 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 Arenicola. 



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 oighou t its whole length with the gaglionic 

 covering of the processes arising from the dorsal and lateral surfaces of 



