268 ANNELIDA. 



vicinity of the neck, where they form fanlike expansions, or ramified 

 tufts, so arranged as to be most freely exposed to the surrounding 

 medium. The mouth, placed at the origin of the tentacular cirri, is a 

 simple orifice closed with a valve-like flap or upper lip, but is unpro- 

 vided with any dental structure. The alimentary canal is generally a 

 simple and somewhat capacious tube that traverses the axis of the 

 body ; but in some species, as in Sabella pavonina, it assumes a spiral 

 course, making close turns upon itself from the mouth to the anal aper- 

 ture, which is always terminal. 



(682.) The branchial organs, in the genus Terebella*, appear under 

 the form of blood-red tufts, proceeding from three separate root-vessels 

 on either side of the occiput. The vessels divide for the most part 

 dichotomously, forming an arborescent bunch of florid blood-vessels ; 

 each ramusculus is enclosed in a delicate cuticular envelope perfectly 

 destitute of cilia, and is, moreover, double that is, composed of an 

 afferent and efferent vessel. Although extremely transparent and 

 attenuated, the cuticular structure embracing these branchial blood- 

 vessels must include some retractile fibres, since each separate ramus- 

 culus may be emptied and rendered bloodless by the compression of the 

 parietes, a provision which frequently exists in many parts of the cir- 

 culating system of the Annelida. 



(683.) The cephalic tentacles of the Terebellae present a problem 

 interesting alike to the physiologist and the mechanician. Prom their 

 extreme length and vast number, they expose an extensive aggregate 

 surface to the agency of the surrounding medium. They consist, in 

 Terebella nebulosa, of hollow, flattened, tubular filaments, furnished with 

 strong muscular parietes. Each of these hollow band-like tentacula 

 may be rolled longitudinally into a cylindrical form, so as to enclose a 

 hollow semicircular space if the two edges of the band meet, or a semi- 

 cylindrical space if they only imperfectly meet. This inimitable mecha- 

 nism enables each filament to take up and firmly grasp, at any point of 

 its length, a molecule of sand, or, if placed in a linear series, a row of 

 molecules. But so perfect is the disposition of the muscular fibres at 

 the extreme end of each filament, that it is gifted with the twofold 

 power of acting on the sucking and on the muscular principle. When 

 the tentacle is about to seize an object, the extremity is drawn in, in 

 consequence of the sudden reflux of fluid in the hollow interior ; by this 

 movement a cup-shaped cavity is formed, in which the object is securely 

 held by atmospheric pressure: this power, however, is immediately 

 aided by the contraction of the circular muscular fibres. Such are the 

 marvellous instruments by which these peaceful worms construct their 

 habitations, and probably sweep their vicinity for food. 



(684.) The inferior aspect of each of these tentacles is profusely 

 clothed with cilia, and this side is thinner than the dorsal. The peri- 

 * Dr. Williams, loc. cit. p. 194. 



