32 



OVUM. 



at one time encysted, and that of the two 

 principal forms distinguished by that observer, 

 the one is the more advanced and the other 

 is the larva. 



From what has already been observed it 

 seems probable that other productions pre- 

 viously described as Entozoa of various kinds, 

 may, in reality, be nurses or larvae of different 

 Distomata; and that many of these maybe 

 brought under their several specific distinc- 

 tions, when the new paths of investigation 

 pointed out by the suggestions of Steenstrup 

 and V. Siebold have been diligently pursued. 



Von Siebold has recently related* a very 

 interesting observation on the remarkable 

 double Trematode animal, discovered and 

 described by Nordmann+, under the name of 

 Diplozoon paradoxum, from which it seems 

 to be ascertained that this double animal is 

 produced by the actual union of two nearly 

 similar simple ones, by a process of partial 

 fusion, which, though much less complete, 

 seems to partake in some degree of the na- 

 ture of conjugation, such as occurs in the 

 Closterium and some of the lower vegetable 

 bodies. The single animal, first described by 

 Dujardin, under the name of DiporpaJ, was 

 observed infesting the minnow (Leuciscus 

 phoxinus)in the same streams with the gudgeon 

 (Gobio fluviatilis). This animal corresponds 

 nearly in form and structure with the half of the 

 Diplozoon, with the exception of its smaller 

 size, and the absence of generative organs. 

 On the side of the Diporpa a projecting sucker 

 exists, and the union between two of these 

 animals which gives rise to the Diplozoon, 

 begins by a mere adhesion of this sucker, 

 which becomes more and more complete, so 

 as at last to lead to that entire fusion and 

 combination of the adjacent parts of the in- 

 testinal canal and some other organs, which 

 has excited so much surprise in the Diplozoon. 

 The development of the genital organs in 

 both of the two united animals succeeds to 

 this union. 



Annelida. Some phenomena in the repro- 

 duction of the Annelida are to be referred to 

 the indirect mode of generation now under 

 consideration. The most of these animals are 

 hermaphrodite ; they are all more or less 

 jointed, or formed in the adult of repetitions 

 of segments of similar structure, the ante- 

 rior and posterior alone differing from the 

 rest. The jointed structure does not exist 

 in the embryo when it first leaves the egg ; 

 but is gradually produced by a process of 

 gemmation, which ma} 1 be styled intervening 

 rather than fissiparous. In the multiplication 

 of these segments the new ones are always 

 formed in the interval between the caudal 

 segment and that which is next to it ; the seat 

 of new production differs therefore in the 

 Annelida and Cestoid worms ; for in the latter 



* Zeitschrift fUr Wissensch. Zool. 1851. 



t Op. cit. 



j Hist. Nat. des Helminthes, 1845. Dujardin had 

 noticed the resemblance between this body and the 

 two parts of the Diplozoon, and had conjectured that 

 it might be in some way the young of the Diplozoon. 



of these animals the new joints are developed 

 at the cephalic extremity, and there is also 

 some difference in its nature, as the multipli- 

 cation of joints in the Tasnia is in some degree 

 truly fissiparous. 



A few of the jointed Annelida have long 

 been known to be subject to another kind of 

 development, by which one or more complete 

 segmented individuals are formed close to their 

 caudal extremity, and spontaneously separate 

 to enjoy for a time an independent life. This 

 remarkable fact was first described by Otto 

 Fred. Miiller, in the small Nais proboscidea*; 

 and Gruithuisen described accurately the same 

 phenomenon in a Nereis or Chaetogaster.f 

 This process was looked upon by these ob- 

 servers as an instance of accidental fissiparous 

 generation ; but it has received a different 

 signification and a greater interest from the 

 more recent researches of Quatrefages and 

 Milne-Edwards. 



The first of these naturalists observed in a 

 number of individuals belonging to the genus 

 SyllisJ, at a certain period of their life, a new 

 individual to be formed at the caudal extremity 

 of each. The part was first marked off by 

 a notch or transverse groove, the form of the 

 parent individual gradually appeared in it, with 

 the head, eyes, the same joints, limbs, &c., 

 and it was ultimately separated by spontane- 

 ous fission. But the resemblance between 

 the original individual and its offspring was 

 chiefly external ; for it was found that while 

 the parent animal continued to exercise as 

 before the functions of nutrition it was not 

 possessed of generative organs ; and, on the 

 other hand, the new individuals seemed de- 

 stined alone to perform the reproductive func- 

 tions, and contained the fully formed sexual 

 organs, while their alimentary canal appeared 

 to become atrophied, and was not employed 

 in the digestion of any newly assumed food. 

 These individuals lived long enough after 

 separation to complete the reproductive pro- 

 cess by the formation of fecundated ova. 



Milne-Edwards observed in the Myrianida 

 fasciata$, a similar, but more numerous gem- 

 miparous production of sexual individuals. In 

 this animal, as many as six new individuals 

 were observed to be formed in gradual succes- 

 sion, one before the other, and between the 

 caudal and terminal segments of the original 

 body. Each one of these new individuals, as it 



* Naturgesch. einiger Wurmarten des Stissen und 

 Saltzigen Wassers, Copenhagen, 1800 ; and in a 

 Nereis, in Zoologia Danica. 



f Nov. Act. Nat. Cur. vol. xi. See also J. Mul- 

 ler's Physiol., by Baly, vol. ii. p. 1424; Owen's 

 Lect. on the Gener. and Develop, of the Inverte- 

 brated Animals, in 1849, in Med. Times, vol. xx. 

 p. 88, where he refers also to observations of Oersted 

 in an Eulosoma, and of Schmidt in a tubicolar An- 

 nelide, called Filograna. 



I Annal. des Scien. Nat. 1844, torn. i. p. 22. Otto 

 F. Miiller had also noticed the phenomenon in the 

 same animal, and described it under the name of 

 Nereis prolifera, in his Zoolog. Danica, vol. ii. p. 15. 



Annal. des Scien. Nat. 1845, torn. iii. p. 170. 

 See also Longet's Traite de Physiologie, torn. ii. 

 part 3rd, p. 47. 



