FAMILIES, GENERA, AND SPECIES OE THE BRITISH MARINE 



ANNELIDS. 



(ANNELIDA POLYCHJSTA, continued.) 



Family XV. — Opheliimi (Ophelracea), Orube, 1851. 



The head is little differentiated, more or less elongated and conical, with ciliated 

 nnchal organs protractile from pits posteriorly. Eyes present or absent. Buccal segment 

 without tentacles, though it has bristles, and a setigerous process, or occasionally none. 

 Proboscis protrusible, patelliform. The body is comparatively short, of two regions, ringed, 

 rounded dorsally, grooved ventrally, occasionally slightly tapered toward each extre- 

 mity (fusiform) and iridescent, the anus having various processes — sometimes valvular. 

 Only the anterior region with dissepiments. Branchise in the form of simple cirri, and 

 anterior or posterior in position, sometimes distributed throughout the entire length. 



Fig. 95. — Transverse section of Polygordius apogon. m, oblique muscle. 



The feet are biramous, not conspicuous, sometimes indistinct or absent, and carry 

 fascicles of simple bristles ; occasionally with a cirriform lobe. 



Inhabitants of sand or mud, in which they burrow. 



The structure of the body-wall in this family presents an interesting series of steps 

 from the condition in Polyojphthalmus to that in Ammotrypane. In Poly ophthalmitis the 

 body-wall approaches that of Polygordius in its general features, as pointed out long ago 1 ; 

 indeed, a section of the latter may well be taken as the initial stage in the series, the 

 oblique muscles (m) being slender, and passing from a little below the mid-lateral region 

 to the ventral nerve-cords (Fig. 95). These muscles, from their slenderness, are unable to 

 affect the contour of the body-wall in this form, else they would by their contraction mark 

 off a ventro -lateral area on each side, as happens in Polyojphthalmus (Pig. 96). Externally 



1 ' Proc. Eoy. Soc. Edinb./ 1873-74, vol. viii, p. 386, and 'Ann. Nat. Hist./ ser. 4, vol. xvi, 1875, 

 p. 369, with 2 text-figs. 



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