ANATOMY AND CLASSIFICATION OF THE ARENICOLIDiE. 479 



In the "ecaudata^' section the commissures are much 

 stouter. They run downwards on each side of the mouth 

 and then backwards, uniting at a distance of two annuli from 

 the first parapodium, so that in all species of Arenicola the 

 origin of the ventral nerve-cord lies in the second segment 

 of the body. 



Branches. — The connectives give rise to nerves (1) to the 

 skin at the level of the interannular grooves through which they 

 pass ; (2) to the otocyst; (3) to the ventral half of the buccal 

 mass. The last nerve constitutes part of the stomogastric 

 system, or, more correctly, it is probably united at the end of 

 the eversible buccal region to the stomogastric system. The 

 nerve to the otocyst deserves a separate description. It is 

 •2 mm. long, and is given off just opposite its otocyst, to which 

 it runs upwards and inwards. Its chief peculiarity lies in the 

 rich collection of ganglion cells, both at the point of origin of 

 the nerve and along the greater part of its course. They are 

 of two kinds. One, large, pyriform, measures "008 mm. in 

 breadth and '016 in length. Their nuclei are "007 mm. in 

 diameter, and have a vesicular nucleus and a distinct nucle- 

 olus. The other kind of cell is smaller, of undetermined 

 shape, and a nucleus '004 mm. diameter. The former occur 

 chiefly on the outer side of the nerve, and do not reach the 

 otocyst itself. The latter are more abundant, and are packed 

 away between the larger cells and occur on the nerve right 

 up to the base of the otocyst. 



This ganglionation of the otocyst nerve is, however, only 

 an expansion of a local thickening of the connectives which 

 underlie the nerve to the body-wall, and this, again, is an en- 

 largement of the continuous ganglionic covering which in- 

 vests the inner side of the connective along its entire length ; 

 but the great development of their ganglion cells in the nerve 

 to the otocyst is peculiai', and not met with in any other 

 peripheral nerves, — at least, not to the same degree. 



