POLYDORA. 



195 



and below the great hooks a small tuft of capillary bristles with winged tips. The body- 

 wall slopes from the upper lateral region to the convex ventral surface. In the centre is 

 the alimentary canal with a powerful series of vertical fibres on each side stretching from 

 the dorsal to the ventral wall. A dense mass of fibres radiates, from the base of the 

 powerful bristles superiorly, upward and outward to decussate with the circular fibres at 

 the outer and upper angle of the body-wall. A still stronger series of fibres radiates 

 ventrally to interdigitate with the ventral longitudinal muscle of each side, and are fixed 

 to the basement tissue of the hypoderm, the inner being nearly vertical, the rest sloping 

 downward and outward, and, in connection with this preponderance of the ventral series, 

 it has to be noted that the curve of the tip of each hook is directed upward. A group of 

 longitudinal fibres is separated from the ventral longitudinal muscle on each side by the 

 strong vertical fibres adjoining the alimentary canal, and passing for insertion into the 

 basement-tissue of the ventral edge. The oblique muscles stretch from the lower border 

 of the setigerous region to the ventral median line, thus crossing in their progress the 



#— so. 



nt. 



>xC. 



Fig. 123. — Transverse section of the posterior region of Polydora ciliata, Johnston, showing the pennate condition of 

 the central muscles, cvm, of the ventral longitudinal series. 



vertical fibres just alluded to. Beneath the insertion of the oblique is the nerve-trunk of 

 each side lying in a thickened area of hypoderm. 



In the posterior region (Fig. 123) the dorsal longitudinal muscles are less developed 

 than, the ventral, which move further outward, the median ventral region being occupied 

 by two separate pennate muscles, cvm. (in transverse section), which form a V below the 

 alimentary canal. These special muscular slips are characteristic of this form, and they 

 extend throughout a considerable region of the body behind the front. A single large 

 neural canal exists at the upper region in the middle of the fused cords. 



The genus Polydora was first clearly described by Claparede * (1861), who, however, 

 placed it under the family Ariciidee partly because no jaws were present. In this account 

 he distinctly outlines all the salient features of Bosc's genus hitherto imperfectly known 

 and described. He compared a Polydora he procured in the Hebrides with that of Bosc 

 from Charleston, in the United States, and appears to agree with the older author in 

 considering that the caudal process was used as a sucker. He again refers to the group 

 in his i Beobach. Anat. u. Entwickel.' 2 



1 ( Recherches Anat./ p. 47 ; see also ' Archiv f. Anat. u. Physiol./ Bd. iii ; p. 542. 

 3 Pp. 36 and 37. 



