ANNULATA. 247 



true blood which is distributed ia vessels over the parietes of these 

 chjious repositories." 



The rythmical elevation and depression of the respiratory scales of 

 the Aphrodite are interesting facts, and we shall afterwards see their 

 homologues taking an important share in the locomotive functions of 

 the higher organised forms of Articulata. 



Most Annelids exude a tenaceous mucus from the skin ; the excre- 

 tory follicles are arranged in curved rows on both the dorsal and 

 ventral aspects of the rings in the leeches and larger earthworms. 



The precise locality of the secretion of the horny or leathery tubes 

 of certain Annelids is not yet understood ; but the tumid ridge around 

 the first segment of the Serpulce, seems to be the formative organ of 

 the calcigerous cells in the production of the shelly sheath. Four 

 yellowish glands, terminating by a common orifice on the first body- 

 segment of the Amphitrite, most probably prepare the mucous cement, 

 by which the particles of sand and shells are attached together to 

 form their neat artificial tubes. The long and delicate cephalic ten- 

 tacles of the Terebellce take an important share, hardly to be suspected 

 at the first glance, in the nidification of these humble sea-worms. 

 " They consist," writes Dr. Williams, "in T. nebulosa, of hollow flat- 

 tened tubular filaments, furnished with strong muscular parietes. The 

 band may be rolled longitudinally into a cylindrical form, so as to in- 

 close a hollow cylindrical space, if the two edges of the band meet ; or 

 a semi-cylindrical space, if they only imperfectly meet. This inimi- 

 table 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 row 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 which the object is securely held by atmospheric pressure; 

 this power 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 pro- 

 bably sweep their vicinity for food."* 



♦ CXC. p. 194. 



R 4 



