198 



CHAPTERS ON EVOLUTION. 



Brittle-stars and Sand-stars (Fig. 107, 5), and exhibiting certain? 

 differences in sfructure from the common Starfishes of our sea- 

 beaches. In their development these Sand-stars and their neigh- 

 bours approach very nearly indeed to that of the Sea-urchins. Their 

 larva is also a " Pluteus," and possesses a limy skeleton ; and it 

 is singular to find that forms so divergent in character as the 

 Sand-starfishes and Sea-urchins should thus resemble each other 



in development. 



The interesting 

 j t \~/ \ *---f J group of the Crinoids, 



ct.M\ ^&^ * r ea . lilies (Fij? ; 



109) well known in 

 a fossil state under 

 the name of " Encri- 

 nites " presents us. 

 with beings that may 

 best be described as- 

 Starfishes borne on 

 stalks. There exists,. 

 however, a well- 

 known free Crinoid in the shape of the Comatula (Antedon} rosacea r 

 or the Rosy Feather-star of our coasts (Fig. 112) ; this form appear- 

 ing in its adult condition as a free, unattached "Starfish" (Fig. 112, a) r 

 but indubitably proving its Crinoid nature, in that it spends the early 

 part of its existence in a stalked condition (), resembling the per- 

 manent state of its neighbour Sea-lilies (Fig. 109). Now, in the 

 development of the Crinoids, we meet with an oval, free-swimming 

 larva, within which a digestive system duly appears. This organism 



FIG. in. DEVELOPMENT OF SEA-URCHIN. 



FIG. ii7. ROSY FEATHER-STAR AND YOUNG. 



in due course attaches itself by a stalk, and the future Crinoid is 



