26 
EMBRYONIC DEVELOPMENT 
The embryos move within the egg, and their body is surrounded by vibrillse, 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 forms 
or shapes very different 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 the 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 the zoological learning of that naturalist, I 
may be permitted to examine critically his brief reasoning. Planarise, in his estima- 
tion, belong to the same natural group as intestinal worms. He saw embryos of Pla- 
narise resembling the polygastric infusoria — Paramecium and Kolpoda;\ie alludes to 
a paper read by himself at the Cambridge meeting of the American Association for 
the Advancement of Science, held in August 1849, in which he showed their iden- 
tity, 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 T the correct- 
ness of Blanchard’s views respecting the Planarise,” I most candidly confess that I 
cannot see the bearing. F or, in admitting it, the same bearing would be true, applied 
to the class of Medusa, and Medusae, on the same ground, would belong to the same 
natural group as the intestinal worms. 
Furthermore, you may 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 
fig. 62, accompanying the present Memoir, and decide which is most Paramecium-like 
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. 
tProc. Amer. Assoc. Adv. of Sc. Second Meeting, 1850, 438. 
