176 



THE ANNELIDES. 



^163. 



Spontaneous transverse fissuration occurs particularly ■with the Abran- 

 chiati,''* but has also been observed with the Neniertini/-* and Branchiati.*^' 



It occurs usually at the middle portion or at the border between the 

 second and third segments of the body. Very often there may be per- 

 ceived at this point, when this process has somewhat advanced, the place 

 where, with the two future individuals, there will be a new fissuration. If 

 the animal has a proboscis, tentacles, or eyes, these organs are developed 

 with the posterior individual beibre its final separation. ^^' 



These animals have no trace of genital organs, while this process of divi- 

 sion lasts. The individuals thus produced, re-divide, and this division 

 continues until a certain time of the year. It then ceases, and genital 

 organs being developed, reproduction takes place by eggs. 



The extreme vulnerability and reproductive power of many Chaetopo- 

 des, give rise to their frequent multiplication by artificial and accidental 

 division. The fragments thus produced arc finally developed, and the mu- 

 tilated animal ultimately regains its lost parts. '^' Some have the power of 

 voluntary division from the least handling of their body,*"' and these sep- 

 arated parts are probably developed to new individuals. 



§ 163. 



Most of the Annelides reproduce by sexual organs, and the few Lumbri- 

 cini which, as just observed, multiply by fissuration, have probably, like their 

 allied species, genital organs at certain seasons of the year.*'* 



The eggs of the Annelides present nothing remarkable ; they are always 

 spherical, and have a chorion and thin vitelline membrane containing a 

 finely-granular vitellus with a germinative vesicle and dot.'-' This 



1 Lumbriculus, Ifais, Chaetogaster a,ni Aeolo- 

 soma. 



'i See Johnston, in the Mag. of Zool. and Bot. I. 

 1837, p. 5o4. 



3 With the Nereifleae. 



4 Fissuration with many species of Nais, has 

 ah-fudy l>t:;n noticed by O. F. Mailer (Naturgesch. 

 einiger Wurm-Arten des siissen und salzigen Was- 

 sers. Taf. II. &c.). For that of Nais proboscidea 

 and Ch'ietoxnsler diaphaiius, see Gruithuiscn, 

 Nov. Act. Acad. Nat. Cur. XI. p. 243, Tab. XXXV. 

 fig. 1, 3 ; X(V. p. 412, Tab. XXV. lig. 2. For tliat 

 of Aeolosoma, see Orsted in Kruyer''s Naturhist. 

 Tidskrilt. IV. I'l. III. lig. 7 ; and for that of Ne- 

 reis prolifera, see Milller, Zool. Dan. II. p. 16, 

 Tab. LII. fig. 6. This last species is a very young 

 Nereis. It is i)n)l)able that many other Branchi- 

 ati multiply in the same way. (^uatrefages {Fro- 

 ricp's neue Not. No. 726, 1845, p. 344) has recently 

 recognised a Si/llis in Nereis prolifera. 



Sars (Faun. litt. &c. p. 87, Taf. X. fig. 18, 19) 

 has observed multiplication by transverse division 

 with Fi/ograna implexa, a young animal detach- 

 ing itself from the caudal end of this Serpula. I have 

 oVjserved a like division with a Protula, a genus 

 allied to 6'er/*u/a. According ti> Milne Edwards 

 (Ann. d. Sc. Nat. III. 1845, p. 180, PI. XI.) a sin- 



* [^162, note 4.] See in this connection, Schultze 

 (Ueber die Fortpflanzung durch Theilung bei Nais 

 proboscidea, in IViegmann's Arch. 1850, p. 293). 

 He has carefully described thL> form of multiplica- 

 tion with this animal, and according to him it is a 

 \i\i9: fissuration, and not a gemmatio7i, iis that of 

 Syllia, described by Frey and Leuckart. See fur- 



gle individual of Myrianida fasciata, which is al- 

 lied to Phyllodor.e, produces six young by as many 

 successively disposed divisions. According to Frey 

 and Leuckart (Beitr. &c. p. 94, Taf. II. fig. 1), 

 there are with Syllis prolifera also, several young 

 developed simultaneously, one after the other, at 

 the caudal extremity.* 



5 See the experiments upon this subject with the 

 Lumbricini by Reaumur, Bonnet, Trembley, and 

 Roesel. Dalyell (^Froriep''s neue Not. No. 331, 

 1840, p. 1) has observed a similar mode of repro- 

 duction with Sabella. 



t> This has been observed by Grube, with Polia 

 delineata (Zur Anat. d. Kiemenwiirmer, p. 58). 

 Meckelia annulata has also the same property. 



1 Aeolosoma. 



2 See fVasner, Prod. Hist, pener. loc. cit. Tab. 1. 

 iig. 9, 10 {Sansuisusa and Nephelis); Stannius, 

 in Miiller's Arch. 1840, Taf II. fig. 1, 2 (Areni- 

 co/a piscatorum) ; Milne Edwards, Ann. d. Sc. 

 Nat. ni, 1845, PI. V. fig. 2, 3, PI. IX. fig. 43, 44 

 (Terebella and Protula); .and Sars, in fVieg- 

 mann''s Arch. 1845, 1. Taf. I. fig. 13 {Polynoi cir- 

 ratn). If the bodies which H. Meckel has figured 

 (Muller's Arch. 1844, p. 481, Taf. XIII, fig. 13- 

 23) as the eggs of Lumbricus terrestris, are really 

 such, wliich I think Ls doubtful, they differ much 



ther Leuckart, Ueber die ungeschlechtlichte Ver- 

 mehrung bei Nais proboscidea, in Wiegm.ann''s 

 Arch. 1851, p. 134, Taf. II. fig. I.-III. ; and Krohn, 

 Ueber die Erscheinungen bei der Fortpflanzung von 

 Syllis prolifera und Autolytus prolifer. Ibid. 1852, 

 p. 66. — Ed. 



