202 THE MICROSCOPE AND ITS KEVELATIONS. 



female are within the appendages of the body. Instead of being set free,, 

 however, into the peri visceral cavity, they are retained within a saccular 

 envelope forming a testis (A, a, a) which fills up the whole cavity of each 

 appendage; and within this the spermatozoa may be observed, when ma- 

 ture, in active movement. They make their escape externally by a pas- 

 sage that seems to communicate with the smaller of the two just-men- 

 tioned rosettes; but they also appear to escape into the peri visceral cavity 

 by an aperture that forms itself when the spermatozoa are mature. 

 Whether the ova are fertilized while yet within the body of the female, 

 by the entrance of spermatozoa through the ciliated canals, or after 

 they have made their escape from it, has not yet been ascertained. Of 

 the earliest stages of embryonic development nothing whatever is yet 

 known; but it has been ascertained that the animal passes through a 

 larval form, which differs from the adult not merely in the number of 

 the segments of the body (which successively augment by additions at the 

 posterior extremity), but also in that of the antennae. At G is repre- 

 sented the earliest" larva hitherto met-with, enlarged as much as ten times 

 in proportion to the adult at B; and here we see that the head is destitute 

 of the frontal horns, but carries a pair of setigerous antennae, a, a, be- 

 hind which there are five pairs of bifid appendages, #, c, d, e, f, in the 

 first of which, Z>, one of the pinnules is furnished with a seta. 'In more 

 advanced larvae having eight or ten segments, this is developed into a 

 second pair of antennae resembling the first; and the animal in this stage 

 has been described as a distinct species, T. quadricornis. At a more 

 advanced age, however, the second pair attains the enormous develop- 

 ment shown at B; and the first or larval antennae disappear, the setigerous 

 portions separating at a sort of joint (G, a, a), whilst the basal projec- 

 tions are absorbed into the general wall of the body. This beautiful 

 creature has been met-with on so many parts of our coast, that it cannot 

 be considered at all uncommon; and the Microscopist can scarcely have 

 a more pleasing object for study. 1 Its elegant form, its crystal clear- 

 ness, and its sprightly, graceful movements render it attractive even to 

 the unscientific observer; whilst it is of special interest to the Physiolo- 

 gist, as one of the simplest examples yet known of the Annelid type. 



599. To one phenomenon of the greatest interest, presented by vari- 

 ous 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 tin-foil), that pass along a considerable number of seg- 

 ments, lasting for an instant only, but capable of being repeatedly ex- 

 cited by any irritation applied to the body of the animal. These scin- 

 tillations may be discerned under the Microscope, even in separate 

 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 observa- 

 tions of M. de Quatrefages, that they are given out by the muscular fibres 

 in the act of contraction. 2 



600. Among the fresh-water Annelids, those most interesting to the 

 Microscopist are the worms of the Nais tribe, which are common in our 



1 Seethe Memoirs of the Author anjl M. Claparede in Vol. xxii. of the " Lin- 

 nsean Transactions," and the authorities there referred to; also a recent Memoir 

 by Dr. F. Vejdovsky in " Zeitschrift f. wiss. Zool.," Bd. xxxi., 1880. 



5 See his Memoirs on the Annelida of LaManche, in " Ann. des Sci. Nat.," Ser. 

 2, Zool., Tom. xix., and Ser. 3, Zool., Tom. xiv. 



