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 out- 

 lines 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 de- 

 rived 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 echinoderms is easily deduced, they yet deviate won- 

 derfully from one another in outward shape and in the 

 construction of the skeleton. The relative value of the 

 difiference between a holothuria and a star-fish, a sea- 

 urchin and a comatula, may be compared to the differ- 

 ence between a mammal and a bird, an amphibian and a 

 fish. Nevertheless, 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 

 margin bent over at both ends like a 

 deck. This border is edged with a 

 continuous row of ciHa, by the agency 

 of which the little boat is moved. A 

 short digestive canal, provided with ^^°- '^' 



a gastric enlargement, is the first essential organ of this 



