STRUCTUKE AND DEVELOPMENT OF ANNELIDA. 637 



gates, and indications of a segmental division become apparent, 

 the head being (as it were) marked off in front, whilst behind 

 this is a large segment thickly covered with cilia, then a nar- 

 rower and non-ciliated segment, and lastly the caudal or tail- 

 segment which is furnished with cilia. A little later, a new seg- 

 ment is seen to be interposed in front of the caudal ; and the 

 dark internal granular mass shapes itself into the outline of an 

 alimentary canal. The number of segments progressively in- 

 creases by the interposition of new ones between the caudal and 

 its preceding segments; the various -internal organs become 

 more and more distinct, eye-spots make their appearance, little 

 bristly appendages are put forth from the segments, and the 

 animal gradually assumes the likeness of its parent ; a few days 

 being passed by the tubicolar kinds, however, in the actively 

 moving condition, before they settle down to the formation of a 

 tube. In places where Annelida abound, free swimming larvse 

 are often to be obtained at the same time and in the same man- 

 ner as those of the Echinoderms (§ 323) ; but to carry out any 

 systematic observations on their embryonic development, the 

 eggs should be searched for in the situations which these animals 

 haunt. To one other pbenomenon of the greatest interest, pre- 

 sented by various small marine Annelida, the attention of the 

 Microscopist should be specially directed ; this is their luminosity, 

 which is not a steady glow like that of the glow-worm or fire-fly, 

 but a series of vivid scintillations (strongly resembling those pro- 

 duced by an electric discharge through a tube spotted with tin 

 foil), that pass along a considerable number of segments, lasting 

 for an instant only, but capable of being repeatedly excited by 

 any irritation applied to the body of the animal. These scintil- 

 lations may be discerned under the microscope, even in separated 

 segments, when they are subjected to the irritation of a needle- 

 point or to a gentle pressure ; and it has been ascertained by the 

 careful observations of M. de Quatrefages, that they are given 

 out by the muscular fibres in the act of contraction.' 



364. Among the fresh-water Annelida, those most interesting 

 to the Microscopist are the worms of the Nais tribe, which are 

 common in our rivers and ponds, living chiefly amidst the mud 

 at the bottom, and especially among the roots of aquatic plants. 

 Being blood-red in color, they give to the surface of the mud, 

 when they protrude themselves from it in large numbers and 

 keep the protruded portion of their bodies in constant undula- 

 tion, a very peculiar appearance ; but if disturbed, they withdraw 

 themselves suddenly and completely. These worms, frointhe 

 extreme transparence of their bodies, present peculiar facilities 

 for microscopic examination, and especially for the study of the 

 internal circulation of the red liquid commonly considered as 

 blood. There are here no external respiratory organs ; and the 



' See his Memoirs on the Annelida of La Mannha, in "Ann. of Nat. Sci." Sir. 2, torn. 

 xix, and Ser. 3, torn. xiv. 



