374 Agassiz on the Relations between Animals 
kingdom, notwithstanding so many striking differences in the plan 
of their structure. This position was assigned to them upon the 
ground of the radiated arrangement of parts around the head, and 
the vascular form of some of their genera, and also upon the sup- 
posed want of a nervous system in all of them. But since the 
discovery of nerves in all of their types, and since the most. inti- 
mate relations have been discovered between them and so many 
other external worms, their complete separation from Annelides as 
a distinct class is hardly recognized now by any modern investi- 
gator. And the necessity of combining the intestinal parasitic 
worms into one great natural group with the other external free 
worms is becoming daily more evident to all, so that whatever po- 
sition be assigned to Annelides in the great type of Articulata, 
Helminths have to follow them, and must therefore be removed 
from the type of Radiata. This point is undisputed now, though 
there may be a difference of opinion as to the propriety of admit- 
ting, to one great class, all Worms, or of subdividing them into 
minor natural groups. 
he third class among Radiata is that of Echinoderms, which 
has been circumscribed within most natural limits since the r 
union of Holothurize and Crinoids, with the common star-fishes 
and true Echini. Whoever is familiar with the embryonic devel- 
opment of Echinoderms, which has been extensively investiga- 
ted of late, will acknowledge an intimate relation between them 
and the other two classes of Radiata, and not be willing to assent 
the proposed separation of Echinoderms as one great type in 
the animal kingdom, placed upon an equal footing with Mollusca, 
and will consider their separation from Polypi and Meduse, as 
proposed by Dr. Leuckardt, rather as a retrograde step, than as an 
improvement upon the general classification of animals, ‘To me 
‘e- 
the type of Radiata, embracing the three classes of Echinoderms, 
Meduse and Polypi, constitutes, in its circumscription illustrated 
above, a most natural group of the animal kingdom, all the mem- 
bers of which are intimately connected by a close uniformity in 
the plan of their structure, but present a remarkable gradation 
of their types in the manner in which this structure is developed 
in each of their classes. And the circumstance that even in the 
higher ones, which contain chiefly free movable animals, we have 
some few representatives attached permanently to the soil upon a 
Polyp-like stalk bearing the radiated animal crown, shows further 
the intimate connection which exists between them all. Radiata 
consist therefore of three classes only, which in their natural gra- 
dation rank as follows: Polypi, lowest, next, Meduse, and high- 
est, Echinoderms. 
As soon as we have removed in this way all the classes or fam- 
ilies which do not strictly belong to the type of Radiata, we cau- 
not fail to perceive at once that all the remaining animals which 
