395 
Dri-ginal Jpiirtes. 
XLIY. — On the Embryology oe the Eciiinodermata. By 
Professor Wyville Thomson. 
Part I. 
1. My object in the present communication is to collate the va¬ 
rious observations which have been made from time to time on the 
embryology and development of the Echinoderms, and to endeavour 
to trace, so far as our limited information will permit, some common 
principle correlating the many modifications of these processes which 
have been observed within the limits of this remarkable group. 
The Echinoderms present in the most marked degree a peculiarity 
which seems to be only imperfectly indicated in the other invertebrate 
sub-kingdoms. This peculiarity consists in the successive develop¬ 
ment from a single egg, of two organisms, each apparently presenting 
all the essential characters of a perfect animal. These two beings 
seem to differ from one another entirely in plan of structure. The 
first, derived directly from the germ-mass would appear at first sight 
to homologate with some of the lower forms of the Annulosa, the 
second, subsequently produced, within, or in close organic connection 
with the first, is the true jEchinoderm. 
The extreme form of this singular cycle, in which the development 
of an intermediate zooid as a separate, independent, living organism, is 
carried to its full extent, is by no means constant throughout the whole 
sub-kingdom, although its existence has been established for all the 
recent orders. In each order it appears to be exceptional, and in cer¬ 
tain cases it is known to be carried to its most abnormal degree in one 
species, while in a closely allied species of the same genus the mode 
of reproduction differs but slightly from the ordinary invertebrate 
type. It seems highly probable that even in the same species, the 
development and independence of the first zooid may be carried to a 
greater or to a less degree under different circumstances. 
2. Eor an organism such as JBipinnaria asterigera which possesses 
all the apparent characters of a distinct animal, which is developed 
from the germ-mass, and which maintains a separate existence before 
the appearance of the embryo, I have proposed the term Pseudembryo ; 
and for all appendages which homologate with the whole or with 
parts of such a pseudembryo, even although they do not assume fully 
the characters of a distinct animal form, I have proposed the term 
pseudembryonie appendages. 
3.1 am inclined for the present to follow the example of many of 
the Continental Naturalists, and to regard the Echinodermata as 
a distinct sub-kingdom, equivalent in value to the Coelenterata on 
one hand, and to the Annulosa on the other. With the latter they 
have certainly in many respects very close relations, but they have 
maintained through the whole series of geological periods a high 
