of a few Annelids i ' 207 



Miiller^s observations first showed the existence of a metamor- 

 phosis in Planaria, while Leuckart and Pagenstecher * subse- 

 quently proved beyond doubt the existence of still more striking 

 changes in Pilidium, of a sort of alternate generation giving rise 

 to Nemertes, as previously suggested by the observations of 

 Miillerf, Busch J, Gegenbaur §, Wagener 1|, and Krohn^ — 

 changes reminding us of a somewhat similar process in the de- 

 velopment of an Echinoderm from a Pluteus. To these evi- 

 dently dissimilar modes of development I still have to add the 

 transformations of Nareda, as shown in a subsequent part of 

 this paper, resembling the usual mode of development of An- 

 nelids; also a sort of retrograde development of a species of 

 Planaria quite analogous to that more fully described in Nareda, 

 where we have a gradual extinction, with advancing age, of veiy 

 distinct articulate features of the young. As in Nareda, we find 

 in this Planaria plainly marked articulations when young, which 

 become less and less distinct with advancing development — a 

 striking contrast to the evolution shown to exist in Planarians 

 by Miiller, and to the usual mode of growth in this family, 

 where the young so early resemble the adult. 



On examining a string of eggs, mistaken at first for those of 

 some naked mollusk, I was surprised to find young Planarice 

 in different stages of growth, with a ramifying digestive cavity 

 somewhat similar to that of adult specimens, but showing be- 

 sides one distinct articulation for each spur of the digestive 

 cavity. The eyes were well developed; and when the young 

 became free, the articulations were still distinct, and the rami- 

 fications of the digestive cavity sufficiently advanced to enable 

 me to determine with tolerable certainty the species to which 

 these young belonged — probably the P/anam angulata, ^IvW.^^ 



In the youngest specimen observed (PI. V. fig. 1) the spurs of 

 the digestive cavity were quite prominent, eleven in number (the 

 first trace of the ramifications of the adult); each spur was 



* " Untersuchungen iiber niedere Seethiere. Pilidium die Larve einer 

 Nemertine," in Arch. f. Anat. u. Phys. 1858, p. 669, pi. xix. 



t " Ueber verschiedene Formen von Seethieren," in Arch. f. ^Vnat. u. 

 Phys. 1864, p. 81. " Bericht iiber einige neue Thierformen der Nordsee/* 

 in Arch. f. Anat. u. Phys. 1846, pi. v. 



X Beobachtungen iiber Anatomic u. Entvvickelung einiger wirbelloser 

 Thiere. Berlin, 1851. 



§ " Bemerkungen iiber Pilidium gyrans, Actinotrocha branchiata und 

 Appendicularia,'" in Zeitschr. f. wiss. Zool. 1853, v. p. 346. 



II " Ueber die Mesotrocha sexocidata von Wilh. Busch," in Arch. f. Anat. 

 u. Phys. 1847, p. 187. 



5[ Archiv f. Anat. u. Phys. 1856, p. 78. "Ueber Pilidium u. Actino- 

 trocha," in Archiv f. Anat. u. Phys. 1858, p. 289. 



** O. F. Muller, * Zoologica Danica.' 



