GENERAL STRUCTURE OF THE NEREIDS. 309 



natural history ; and as for metamorphoses, the zoo- 

 logist could supply him to his heart's content. How- 

 ever, poetry is not our province ; we must return to 

 the broad beaten track of truthful narrative, and in- 

 troduce the Nereids to the reader, such as he will 

 find them in his own aquarium. 



The body of these worms (PL VI. fig. 7) consists 

 of a consecutive series of rings, all of which, with the 

 exception of the anterior segments, that constitute 

 the head, seem to be repetitions of each other, only 

 differing in size as they taper gradually towards the 

 tail. Upon every one of these rings several external 

 appendages are observable, situated on each side of 

 the body. The uppermost of these appendages is 

 frequently a tuft of branching filaments, of a bright 

 red or crimson colour in the living animal ; in other 

 cases it consists of a single stem, to which lateral fila- 

 ments are attached ; or else in some species these 

 organs consist of mere flattened lamellae, formed by 

 processes of the skin ; but whatever may be the shape 

 of these appendages, their nature and office are the 

 same ; they are almost entirely made up of the rami- 

 fications of bloodvessels, and being constantly im- 

 mersed in the surrounding water, the blood flowing 

 through them is effectually exposed to the influence 

 of the air contained therein, and respiration thus 

 adequately provided for. These tufts therefore con- 

 stitute so many pairs of gills or branchiae, and from 

 the circumstance of their being situated upon or near 

 the back of the animal, this group of Annelidans is 

 distinguished by the name of Dorsibranchiate (dor sum, 

 the back, and branchia, a gill). 



