we 
and the Elements in which they live. 373 
If we next consider the Polypi we find them constituting an- 
other main group and most natural class, to which indeed some 
heterogeneous types have been annexed; upon the removal of 
these however that class constitutes a very natural division of 
the type of Radiata among which they form the lowest class. 
The natural groups which require to be removed from Polypi are, 
—first, the so-called Hydroid Polypi, which, though truly radiated 
animals, do not belong to this class, but are, as I have shown from 
their structure, and as might long ago have been inferred from 
their development, true members of the class of Meduse, among 
which they constitute a type of stalk animals, as crinoids among 
star-fishes.* 
The Bryozoa also are not constructed upon the plan of Radiata, 
as has long been shown by Milne Edwards and others. Their 
true position is among Mollusca, and embryonic investigations 
upon Ascidia have satisfied me that Bryozoa, compound, and sim- 
ple Ascidia, form a natural series of well connected types leading . 
to'the true Acephala among ordinary Mollusca, among which Bry- 
ozoa will form a natural group of compound animals, bearing the 
same relation to the ordinary bivalve shells, that common corals 
ar to the simple Actinie and Fungie. Though the doubts en- 
tertained about the Foraminifera among Bryozoa, would not affect 
at all the points under discussion, I may as well state at once, 
that I have arrived at the conclusion that Foraminifera constitute 
the lowest type of Gasteropoda, and exemplify under permanent 
rms the state of division of their germs in their embryonic de- 
velopment. Thus circumscribed, the class of Polypi constitutes 
a very natural group containing only animals of an identical radi- 
ated structure, the organization of which is at present very satis- 
factorily known. 
The class of oe has been from the beginning so well 
‘characterized, and circumscribed within so natural limits, that it 
has undergone since its establishment only slight modifications by 
the removal of some few genera: and after the position of the 
so-called Hydroid Polypi among them shall have been generally 
_ acknowledged, I believe it will undergo scarcely any new changes 
In its extension, though we may still expect extensive improve- 
ments, which are indeed very much heeded, in the characteristics 
and internal arrangement of their natural families. Considering 
their structure, the Medusxe rank immediately above Polypi. 
The Intestinal Worms have long been placed among Radiata, 
and considered as a natural class in this great type of the animal 
ie 
lypi, read before the American Association 
, Science, held in Cambridge, August, 1849; also my lectures 
Fa comparative embryology, delivered before the Lowell Institute, Dec. 1848, and 
