NAIS 



955 



observer ; whilst it is of special interest to the morphologist as one 

 of the simplest examples yet known of the Annelid type. 



To one phenomenon of the greatest interest presented by various 

 small marine Annelids 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 produced by an electric 

 discharge through a tube spotted with tinfoil), 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 scintillations may be discerned under 

 the microscope, even in separate segments, when they are subjected 

 to the irritation of a needle-point or* 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. 1 



Among the fresh- water Annelids 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 colour, 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 undulation, a very peculiar appearance ; 

 but if disturbed they withdraw themselves suddenly and completely. 

 These worms, from the extreme transparency of their bodies, present 

 peculiar facilities for microscopic examination, and especially for the 

 study of the internal circulation of the red liquid commonly con- 

 sidered as blood. There are here no external respiratory organs, and 

 the thinness of the general integument appears to supply all needful 

 facility for the aeration of the fluids. One large vascular trunk (dorsal ) 

 may be seen lying above the intestinal canal, and another (ventral) be- 

 neath it, and each of these enters a contractile dilatation, or heart- 

 like organ, situated just behind the head. The fluid moves forwards 

 in the dorsal trunk as far as the heart, which it enters and dilates ; 

 and w T hen this contracts it propels the fluid partly to the head and 

 partly to the ventral heart, which is distended by it. The ventral 

 heart, contracting in its turn, sends the blood backwards along the 

 ventral trunk to the tail, whence it passes towards the head as 

 before. In this circulation the stream branches oft' from each of 

 the principal trunks into numerous vessels proceeding to different 

 parts of the body, which then return into the other trunk ; and 

 there is a peculiar set of vascular coils, hanging down in the peri- 

 visceral cavity that contains the corpusculated liquid representing 

 the true blood, which seem specially destined to convey to it the 

 aerating influence received by the red fluid in its circuit, thus 

 acting (so to speak) like internal gills. The Naiad worms have 

 been observed to undergo spontaneous division during the summer 

 months, a new head and its organs being formed for the posterior- 

 segment behind the line of constriction before its separation from 



1 See his memoirs on the Annelida of La Manche in Ann. des tici. Nat. ser. ii. 

 Zool. torn. xix. and ser. iii. Zool. torn. xiv. ; and Professor Mclntosh in Nature, 

 xxxii. p. 478. 



