in some Chatopod Annelids. 329 



limited series of ring-formations proceed from the point of fis- 

 sion*. The resemblance between the two is perfect; and as the 

 fixed point is not related to specializations of the alimentary 

 tube as I at first supposed, and is in Stylaria proboscidea, where 

 it occurs by " parting," four hook combs back of the mouth, as 

 it is in Nais and Dero, where it occurs by budding, while in S. 

 longiseta it is six hook combs back, the genetic relations of the 

 two processes, in these genera at least, are completely one. But, 

 as I have already said, though the distinction appears unessen- 

 tial in the genera I have examined, the terms are convenient, 

 and, as merely descriptive terms, are used here. 



The " commencement of fission" was observed in a large pro- 

 portion of the buds given off from the individuals of Stylaria 

 and Nais which were under observation, and the result is given 

 in the following table. 



Stylaria. Nais. 



Between 12— 13th combs in none. Between 17-1 8th combs in 3 



„ 13-14 „ 2 „ 18-19 „ 3 



„ 14-15 „ 12 „ 19-20 „ 4 



15-16 „ 9 „ 20-21 „ 3 



„ 16-17 „ 1 „ 21-22 „ 3 



It is evident that fission does not begin at a fixed point ; nor 

 have I been able to discover any relation between the place of 

 its occurrence and the time of the year, temperature, &c. 



Nov/, while fission may take place by gradual reduction of the 

 Naid Stylaria, between the 10th and 11th hook combs, the com- 

 mencement of fission has not been known forward of the 13th. 

 In Nais rivulosa, also, fission has been observed as far forward 

 as the 15-1 6th, while its commencement has not been noted an- 

 terior to the 17th hook combs. This is all the difference between 

 the commencement of fission and continued fission, notwith- 

 standing the fact that, whether the former is introductory to a 

 series of " partings " or of " buddings," its bud resembles that 

 produced by what I shall call the " renewal of fission." 



That the " renewal of fission," in a Naid elongated after re- 

 duction by fission, is a somewhat peculiar form of fission would 

 hardly have been known from observations on Stylaria alone f. 



* There is an interesting analogy between this process in the Naids and 

 the embryonic growth of Terebella, as described by Milne-Edwards. He 

 has remarked (" Obs. sur le Developpement des Annelides," Ann. des Sc. 

 Nat. 1845, ser. 3. t. iii.) that the first defined part is not the cephalic nor 

 the anal, but the oesophageal, and that growth takes place both anterior 

 and posterior to this by succession from before backward. Other specula- 

 tions and analogies suggest themselves here, but are, in our present know- 

 ledge, wholly premature. 



t Yet Midler seems to notice these two forms of fission, and says that, 

 "though at first view different, they are fundamentally the same." {Op. 

 cit. p. 38.) 



Ann. $ Mag. N, Hist. Ser. 3. Vol si. 22 



