324 EMBRYONIC DEVELOPMENT 



in that respect between Flanocera ellipiica and Eolis gynmoia, in the cases in which 

 the yolk divides into three spheres instead of four. 



The vitellus, as a whole, transforms itself into an embryo ; there being no embryonic 

 layer distinct from any other portion of its mass. 



The embrj'os move within the egg, and their body is surrounded by vibrilla?, both 

 in Planarians and Nudibranchiata, and during the earliest period of their existence 

 they resemble each other most. 



Planarians and Gasteropods undergo a larval life, during which they assume for m.s 

 or shapes very diiferent from those of the adults. 



There is now the state of chrysalis which has not yet been observed among Gastero- 

 pods. 



§ 2. An opinion adverse to the views which I now entertain respecting ihe systematic 

 position of Planarians has been published* under the heading of "Zoological Notes 

 from the correspondence of Prof. Agassiz." 



With all the respect which I have for his zoological learning, I may be permitted to 

 examine critically his brief reasoning. Planarise, in his estimation, belong to the 

 same natural gro7(p d.%\niQ^i\ndX worms. He saw embryos of Planaria? resembling 

 the polygastric infusoria — Para7necium and Kolpoda; he alludes to a paper read by him- 

 self at the Cambridge meeting of the American Association for the Advancem.entof 

 Science, held in August 1849, in which he showed their identity, whilst, according 

 to the records of that meeting,| that paper was neither read nor printed; the title 

 only appeared. 



The dismemberment of the Class of Infusoria, discussed in the "Zoological Notes,'' 

 is foreign to my subject. 



Now that that dismemberment, even if rational, should tend " to show the correct- 

 ness of Blanchard's views respecting the Planarise," I most candidly confess that I 

 cannot see the bearing. For, in admitting it, the same bearing would be true, applied 

 to the class of Medusa, and Medusse, on the same ground, would belong to the same 

 natural group as the intestinal worms. 



Cast a glance at page 135 of the "Principles of Zoology" of the same author, 

 compare the figure he gives there of a young Medusa, with tig. 62, accompanying 

 the present Memoir, and tell us after that which is most Paramecium-Jike of the two. 



§ 3. In the third Part of these Researches I propose to illustrate the anatomical and 

 physiological grounds for placing the Nemertians and Planarians in the division of 

 Mollusca, when I shall also examine and discuss the opinion and observations of authors 

 upon the same subject. 



* Amer. Jour, of Sc. and Arts, Second Series, vol. xiii. 1852, 425. 

 iProc. Amer. Assoc. Adv. of Sc. Secood Meeting, 1850, 438. 



