194 REPORT 1851. 



the circulating powers exists in various parts of the circulating system of the 

 Annelida. It may be affirmed generally, that in all true Terebellce the 

 branchiae occur under the character of naked uncilicUed blood-vessels 

 restricted to the occipital rings of the body. 



In Terebella nebulosa they form thick rich tufts ; in T. conchilega they 

 are less prominent ; in the small species they are scarcely visible, but in all 

 the structure is identical. 



The cephalic tentacles in the Terebellce constitute, unquestionably, auxiliary 

 organs of respiration, not for the aeration of the blood-proper, but for that 

 of the peritoneal fluid, by which they are freely and copiously penetrated. 

 They present a problem interesting alike to the physiologist and the mecha- 

 nician. From their extreme length and vast number, they expose an exten- 

 sive aggregate surface to the agency of the surrounding medium. They 

 consist, in T. nebulosa, of hollow flattened tubular filaments, furnished with 

 strong muscular parietes. The band may be rolled longitudinally into a 

 cylindrical form, so as to enclose a hollow cylindrical space, if the two edges 

 of the band meet, or a semi-cylindrical space if they only imperfectly meet. 

 This inimitable mechanism 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 roio of molecules. But so perfect is the disposition of the muscular fibres 

 at the extreme free 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 wJiich the object is securely held by atmo- 

 spheric pressure ; this pover is however immediately aided by the contraction 

 of the circular muscular fibres. Such then are the marvellous instruments 

 by which these peaceful worms construct their habitation, and probably 

 sweep their vicinity for food. The inferior aspect of each of these tentacles 

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

 peritoneal fluid, which is so richly corpusculated, and which freely enters the 

 hollow axes of all these tentacles, is thus brought into artful contact with 

 the surrounding water. To deny to such a mechanism the express design of 

 aerating the organic fluid by which they are distended, were indeed to argue 

 against the strongest probability. The minute blood-vessel which runs in 

 the hollow axial space along the whole extent of the filament is so dispro- 

 portionately small in comparison with the volume of the peritoneal fluid by 

 which this space is filled, that the former cannot reasonably be supposed to 

 share in the respiratory function of these organs. In addition to the two 

 important uses already assigned to the tentacles in the Terebella, they con- 

 stitute also the leal agents of locomotion. They are first outstretched by 

 the forcible injection into them of the peritoneal fluid, a process which is ac- 

 complished by the undulatory contraction of the body from behind forwards ; 

 they are then fixed, like so many microscopic cables, to a distant surface, 

 and shortening in their lengths, they haul forwards a step or two the helpless 

 carcass of the worm. 



In T. conchilega, the cephalic tentacles are inferior to those of the former 

 species in number and size ; they are also difl'erently configurated. They 

 approach the prismatic in outline ; in transverse section they present a trira- 

 diate shape. In minute structure, mechanism of action and uses, they 

 coincide in the most exact manner with the tentacles of T. nebulosa. It is 

 not a little curious that in the Terebella, these organs, which are homologous 

 with true cirri, should be so richly provided with vibratile cilia, while the 

 true- blood branchiae are entirely destitute of these motive organisms. Nothing 



