20S !Mr. A. Agassiz on the Young Stages 



placed in a distinctly marked transverse ring. The two ante- 

 rior and posterior rings were much larger than the others. In 

 this stage the young Flanaria scarcely answers to its name; it 

 is almost cylindrical, and only slightly compressed. In fig. 2 

 the processes are larger and more distinctly developed, and the 

 young worm has become considerably flattened. It seems 

 scarcely necessary to refer to the opinion advanced by Girard*, 

 that the Planarians are naked Gasteropods, 



On the Adult of Loven's Annelid Larva (Nareda, Gir,^)f. 



Although Loven was the first to publish observations on the 

 development of Annelids proper, as early as 1842 {, when he 

 traced the development of an Annelid, supposed at the time to 

 be the larva of some iVerm-like animal, yet up to the present 

 day his observations have not been confirmed, in spite of the 

 many memoirs we now possess on the metamorphosis of seve- 

 ral families of true Annelids. Milne-Edwards, who followed 

 closely upon Loven with a most exhaustive history of the deve- 

 lopment of Terebella^, laid the foundation of generalizations 

 on the mode of formation and norm of succession of rings in 

 the young Annelids, which subsequent observations have com- 

 pletely confirmed. These were somewhat diff'erent from what 

 would seem to be logically deduced from the observations of 

 Loven ; so that it is of considerable interest to have the obser- 

 vations of the latter repeated, to show that the development of 

 this larva does not diff'er very materially from the general mode 

 of evolution observed in other Annelids. 



The large disk of the anterior extremity in Loven's larva was 

 regarded by Milne-Edwards as simply due to the distention 

 of that portion of the young Annelid, similar to what he had 

 often observed in some of the younger stages of Terehella 

 while in motion. Larvae with similar disks have since been 

 observed by Sars, Busch, Miiller, and Claparede, which are 

 known to be the young of Polynoe. It was therefore, to judge 

 from the general resemblance of these larvae, most natural to 

 associate Loven's larva with those of Polynoe^ as has been 



* '* Researches upon Nemerteans and Planarians. — I. Embryonic Deve- 

 lopment of Planocera elliptica," in Journ. Acad. Nat. Sciences, Phil. 

 1854. " On the Development of Planocera elliptica," in Proc. Bost. Soc. 

 N. H. iii. p. 348. 



t Charles Girard, in ' Synopsis of Marine Invertebrates of Grand Manan/ 

 by W. Stimpson, in Smithson. Cont. 1853. 



X " Jagttagelse ofver metamorfos hos en Annelid," in K. Vet. Akad. 

 Handl. Stockholm, 1840, p. 93; Archiv f. Natm-g. 1842, i.p.302; also in 

 Ann. d. Scien. Nat. se'r. 2, 1842, xviii. p. 288. 



§ *' Observations sur Iq developpement des Annelidas," in Ann. d. Scien. 

 Nat. 1845, iii. p. 145. 



