DEVELOPMENT OF ECHINODERMS. 197 
similarity of derivation. As we already know, C. E. 
v. Baer first demonstrated that the members of the 
great divisions of the animal kingdom agreeing in the 
outlines of their organization testify their coherence by 
a special “type of development.” This fact was always 
looked upon as self-evident, although, if it were not 
derived from descent, it would be the greatest miracle. 
This is therefore the place for us to review some of the 
fundamental forms of development which we partially 
considered in the third chapter, and at the same time 
to elucidate the meaning of these types with the aid of 
the doctrine of derivation. 
We will take the Echinoderm as our first example. 
Although from the anatomical comparison of a crinoid, 
a star-fish, a sea-urchin, and a sea-cucumber or holothuria, 
the close kindred of these various divisions of echino- 
derms is easily deduced, they yet deviate wonderfully 
from one another in outward shape and in the construc- 
tion of the skeleton. The relative value of the difference 
between a holothuria and a star-fish, a sea-urchin and a 
comatula, may be compared to the difference between 
a mammal and a bird, an amphibian and a fish. Never- 
theless, with some few exceptions which have a special 
meaning, these various echinoderms leave the egg in a 
larval state almost identical. The larva (Fig. 12) is 
boat-like in form, with a curved mar- 
gin bent over at both ends like a 
deck. This border is edged with a 
continuous row of cilia, by the agency 
of which the little boat is moved. A Re 
short digestive canal, provided with FIG. m2 
a gastric enlargement, is the first essential organ of this 
