58 



STARFISH GALLERY. 



Echinoderms are of great geological age, and were very abundant 

 in earlier periods of the world's history. Two groups (the Blastoids 

 and Cystids) have completely disappeared, and the Stalked Crinoids 

 (Lily-Encrinites) are far less common than they used to be. Echi- 

 noderms are now found in all seas, and extend to great depths of 

 ocean ; many of the species have exceedingly wide areas of distribu- 

 tion, and most are characterized by their gregarious habits, a large 

 number of specimens of a single species being generally obtained 

 by the dredge. They are most abundant in the tropical seas. 



Most Echinoderms lay their eggs in the water, where the larvae 

 are developed and swim about freely ; but in a few (Hemiaster, 

 Ophiacantha vivipara, and others) the young do not pass through 

 any metamorphosis, the eggs being received in special pouches of 

 the body of the parent, in which they are hatched. The free- 



Fig. 37. 



Fig. 38. 



Developing larvae. 

 Fig. 37. Pluteus. Fig. 38. Bipinnaria. 



swimming larvae of the other Echinoderms pass through a series of 

 remarkable changes (figs. 37 and 38) ; these are illustrated by the 

 twelve models of various forms of larva? exhibited in Case 7b; in 

 Case 7 a is a set of models showing in detail the changes under- 

 gone by a single species (Asterina gibbosa). A portion only of the 



