POLYDORA. 



193 



Feet biramous, with dorsal and ventral bristles anteriorly, dorsal bristles and ventral 

 hooks posteriorly. 



In Polydora eiliata the body-wall anteriorly is characterised by the great development 

 and bifid nature of the median ridge of cuticle, hypoderm, and muscular fibres, which 

 is flanked on each side by a prominent process of the hypoderm. In transverse section 

 of the snout a little behind the tip, there is on each side of the dorsal process a large 

 rounded (lappet-like) lobe which extends downward to the oral aperture. Externally is 

 the cuticle, then a thick layer of hypoderm, having internally a series of circular fibres, 

 which come from the dorsal arch in the form of a loop at each side and pass to the ventral 

 border, curve inward and then proceed upward over the pharynx to its dorsal region. A 

 well-marked series of longitudinal fibres lines the outer division of the loop (Fig. 120) in 

 the form of fasciculi, and subsequently each merges into the ventral longitudinal muscle of 

 the side. These longitudinal fibres abut on the coelom. Then the section presents a 

 kind of X-shaped central region, the tips of the X being prolonged horizontally so that 



_.vm. 



._Jfr 



Fig. 120. — Transverse section through the month of Polydora eiliata, Johnston. 



the whole section resembles a figure of oo, the lateral ccelomic spaces being sheathed in 

 muscular fibres. The dorsal arch of transverse fibres cuts off the nerve-ganglia in the 

 hypodermic process, and in the space which forms therein is a small group of longitudinal 

 fibres. 



Behind the foregoing a transverse dorsal layer (part of the circular) is found beneath 

 the central and now solid hypodermic process (Fig. 121) next the dorsal longitudinal muscles, 

 the fasciculi of which, in the middle line, are directed downward and outward, whilst the 

 outer are directed downward and inward. The pharynx forms about a third of the area 

 in section as a large organ with folded walls, numerous blood-vessels on each side, and 

 supported by strong bands of muscular fibres chiefly vertical in direction. The longi- 

 tudinal, ventral, and oblique muscles are now largely developed, each of the oblique having 

 the nerve-cord below its ventral attachment, and they are still widely separated. The upper 

 lateral regions have the feet with their bristles, branchiae, lateral lamellae, and numerous 

 blood-vessels. 



The fifth bristled segment bears the remarkable hooks characteristic of the genus, 

 besides bristles with spear-shaped tips and minute fascicles of the ordinary structure. 

 Immediately in front of the hooks the body-wall externally has beneath the hypoderm a 

 circular muscular coat, which is thin at some parts but greatly developed at others. Dorsally 



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