292 ELEMENTS OF COMPARATIVE ZOOLOGY. 



bilateral larva, and in its development the young goes 

 through most wonderful changes. The Echinoderms are 

 all marine. The group is subdivisible into five classes: 



CLASS I. HOLOTHURIDEA (SEA-CUCUMBERS). 



The Holothurians are cylindrical Echinoderms, with 

 mouth and vent at the end of the body, and usually with 



FIG. 130. Sea-cucumber (Pentacta frondosa). From Emerton. 



the ambulacra scattered over the surface in such a way as 

 to make the comparison with a cucumber most apt. Around 

 the mouth is a circle of tentacles (in reality enormously 



