308 On the Young Stages of a few Annelids. 



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 

 very 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 Planarise 

 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 Planaria angulata Mull.* 



In the youngest specimen observed, 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 

 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 Planaria 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 



* MtkLER, 0. F. Zoologia Danica. 



