396 
ORIGINAL ARTICLES. 
degree of importance and at the same time a singular compactness 
as a natural group, and consistency in the maintenance of their leading 
characters. They have passed through a wonderful series of modifi¬ 
cations, but their range of modification has been confined within 
narrow limits. 
4. Although regarding the whole Echinoderm group as a sub¬ 
kingdom, its sub-divisions, though singularly well defined, are so 
closely allied in all important structural characters, that I should hesi¬ 
tate in assigning to any of them, at all events to those of the recent 
forms, more than an ordinal value. Among the fossil groups the Cysti- 
deans might form an exception were we sufficiently well acquainted 
with their structure to pronounce with certainty on their affinities. 
It is probable, however, that this group may have attained its full 
development during remote periods whose records have been utterly 
obliterated. 
5. The recent orders are the Aster idea, the Echinidea , the Opliiu- 
ridea , the lloloihuridea , and the Crinoidea. 
In this classification there is but little serial degradation. The 
first four orders may be regarded as nearly parallel, but in each 
order while the external characters usually remain permanent and 
well marked, there is a wide range in structural differentiation. 
Among the Crinoids there is so great a tendency to diffusion and 
to the vegetative repetition of parts, that I think we are entitled to 
place them at the foot of the series. 
As the Asteridea present well-marked examples of all the princi¬ 
pal modifications of the reproductive process which have been observed 
among the Echinodenns, I shall commence with an analysis of what 
is as yet known with reference to the Embryology of this Order. 
I.—The “ Bipinnaria” Psetjdembryonic eorms. 
6. In the year 1835,* in a remarkable little book which contained 
the germs of many valuable discoveries with reference to the Em¬ 
bryology of the Invertebrata, Professor Sars described under the 
name of Bipinnaria asterigera a singular organism found swimming 
in the sea off the coast of Norway. At that time Prof. Sars was 
unaware of its true relations, and imagined that it might homologate 
with the ciliograde medusae, but in 1844,f in an admirable paper 
describing another form of Echinoderm-development, he suggested 
that Bipinnaria might be a starfish in an early stage of development, 
provided with a large swimming apparatus. 
7. In 1847J Messrs. Koren and Danielssen published a short 
paper on the structure of Bipinnaria asterigera (Sars), in which they 
thoroughly established the truth of this suggestion. In October, 
1846, the harbour of Bergen was visited by multitudes of these 
* “ Beskrivelser og Iagttagelser over nogle ved den Bergenske Kyst levende 
Dyr.” Bergen. 1835. 
f Wiegmann’s Archiv. part 2, 1844. 
j Zoologiske Bidrag, Bergen. 1847. 
