194 



POLYDOEA. 



a powerful series of fibres spreads outward from the middle line on each side, some 

 becoming continuous with the circular coat, others passing obliquely outward and down- 

 ward to the superior bristle-bundle. Interiorly a strong band lies over the nerve-trunks 

 and forms an external investment to the ventral longitudinal muscles. The oblique 

 muscle comes from the lower bristle-bundle, and joins the circular over the nerve-trunks 



Fig. 121. — Transverse section behind the snout of Polydora ciliata, Johnston, showing the massive proboscis and the 



separate nerve-cords. 



after passing the vertical bands. The dorsal longitudinal muscle forms a great mass at 

 each side, and the fibres of the vertical muscles interdigitate with it, and these are greatly 

 developed at the sides of the oesophagus. The same muscles traverse the ventral longi- 

 tudinal muscles in the compartments formed for them by the circular and oblique bands. 

 The size of the ventral is less than that of the dorsal longitudinal muscles. A somewhat 



Crrv 



/m,. 



vrrv 



nc. 



Fig. 122. — Transverse section of the body-wall through the fifth segment of Polydora ciliata, Johnst. few. Muscles 



of the bristles. 



strong group of longitudinal fibres lies within the ventral transverse band. Each fascicle 

 of the ordinary bristles has a V-shaped series of fibres extending from the base of the tuft 

 to the lateral wall, and interdigitating with those from the transverse and other muscles of 

 the region. 



As soon as the powerful hooks of the fifth segment appear, the central area, from 

 the alimentary canal to the body-wall, is occupied by their muscular apparatus (Fig. 

 122, bm.). The median hypodermic ridge of the dorsum ceases with the previous segment. 

 Dorsally the fifth segment is flattened and rounded at the upper angle, and it has above 



