o/ Arenicola piscatorum. lifl 



tion the vitelline membrane retains its form of a simple vesicle, 

 but afterwards unites with the surface of the embryonal cells, 

 forming the skin of the embryo, and acquiring cilia on its outer 

 surface. And this statement acquires more force from those of 

 O. Schmidt with regard to the development of Amphicora [Fa- 

 bricia) sabella"^. This little Annelide, which has been classed 

 amongst the Cephalobranchiate worms, although, according to 

 Schmidt, it bears its branchiae on its tail, lays its eggs in the 

 tube which it inhabits, but which it then quits. Their develop- 

 ment resembles that of Exogene and Cystonereis, and in the 

 course of it the vitelline membrane becomes converted into the 

 skin of the embryo, as in Hermella. New observations must 

 prove the reality of the occurrence of such a participation of an 

 egg-capsule in the formation of the embryo, which would differ 

 from all known modes of animal development. 



If we attempt, in conclusion, to refer the numerous an^ vari- 

 ously formed Annelidan larvae hitherto observed to a few typical 

 forms, in order to facilitate the examination of their further 

 metamorphoses, and acquire an approximate knowledge of the 

 mode of development common to all, the nature of the ciliary 

 coat, and the number and arrangement of the bands of cilia 

 when such occur, present us with a constant character applicable 

 to this purpose, as has already been pointed out by others. 



In this way Buschf distinguishes two groups of Annelidan 

 larvse, — one, for which Loven's larva J serves as the type, 

 possessing a circlet of cilia at each end of the body (the anterior 

 generally situated between the eyes and the mouth), between 

 which the segments of the worm are afterwards developed. These 

 have been subsequently named Telotrochae by J. Miiller (Archiv, 

 1855, p. 12) ; and besides Loven's larva, the destiny of which is 

 unknown, this series includes a portion of Busch^s larva?, which 

 also cannot be referred to their parents, the larvse of Polynoe 

 (Sars), Nereis (Busch, /. c. tab. 9. fig. 11), Terebella, Protula, 

 and Arenicola. Perhaps also the young Hermellce may be re- 

 ferred to this position, as Quatrefages thinks (Ann. des Sc. Nat. 

 3 ser. X. p. 189), although, according to the description and 

 figure, they are destitute of a hinder circle of cilia. 



The second group established by Busch is that of the Meso- 

 trocJifB, with a simple or double wheel-like organ situated in the 

 middle of the body. To this belong all the larvse to which the 

 generic name of Mesotrocha has been applied, one of which, the 

 M. sexoculata, as already stated, is now ascertained to be the 

 young state of a Chcetopterus. ^^^ 



* Neue Beitrage zur Naturgeschichte der Wiirmer, p. 29. ^^"^ 



t Beobachtungei), &c., pp. 57, 62. 

 X Wiegmann's Archiv, 1842, p. 302. 



