THE EVIDENCE FROM DEVELOPMENT. 



197 



1 10, B, a b\ and the curious spectacle is soon beheld of the form of the 

 young Starfish growing within and absorbing the materials of which 

 the Bipinnaria-body is composed. So that when development is 

 completed, the Bipinnaria's substance has become appropriated by the 

 new and secondary formation, which latter duly appears as the true 

 Starfish, destined, after ordinary growth, to assume the adult form. 



The study of a Sea-urchin's early life-history reveals a striking 

 similarity to the development of a 

 Starfish. The embryo Sea-urchin, 

 "in escaping from the egg" (Fig. 

 n i, A B), says Agassiz, "resembles 

 a Starfish embryo, and it would 

 greatly puzzle any one to perceive 

 any difference between them. The 

 formation of the stomach, of the 

 oesophagus (or gullet), of the intes- 

 tine, and of the water-tubes takes 

 place in exactly the same manner 

 as in the Starfish, the time only at 

 which these different organs are 

 differentiated not being the same." 

 But at a later stage the young Sea- 

 urchin develops a different phase 

 and form from those of the Star- 

 fish. It appears as a curious body, 

 shaped somewhat after the fashion 

 of a painter's easel and formerly 

 named Pluteus (Fig. 1 1 1, c), under the idea that it represented an 

 adult and distinct being. Within this Pluteus, a skeleton of limy 

 rods is developed, and a digestive system is also formed. Then 

 succeed the final stages in 

 development. The body of 

 the Pluteus is absorbed by 

 the future Sea-urchin (Fig. 

 in, D), which, as in the 

 Starfish, is formed within 

 and from the substance of 

 this larva with this differ- 

 ence, that a portion of the 

 Pluteus is generally cast off 

 as useless material, whereas 

 in the Starfish the whole 

 larva was utilised in the 

 manufacture of the perfect 

 form. There exists a second group of Starfishes, including the 



FIG. 109. CRINOID. 



FIG. no. LARV.E OF STARFISH. 



