DEVELOPMENT OF ECHINODKRMS. 197 



similarity of derivation. As wc already know. C. V. 

 V. Baer first demonstrated that tlie members of tlie 

 great divisions of the animal kingdom agreeing in tiic 

 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 ha\e a special 

 meaning, these various echinoderms lca\ e 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 

 short digestive canal, provided with 

 a gastric enlargement, is the first essential orcran of this 



