EOHINODERMA. 



Ill 



are developed and swim about freely ; but iu a few {Hemiast&r, 

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

 any metamorphosis, for the eggs are placed in special pouches of 

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

 swimming larvse of the other Echinoderms pass through a series 

 of remarkable changes (Figs. 4 and 5) ; these are illustrated by the 



Pluteus. 



Developing larvse. 



twelve models of various forms of larvse exhibited in Case 36 ; in 

 Oase 35 is a set of models showing in detail the changes under- 

 gone by a single species {Asferina gihiosd). A portion only of the 

 body of the larva is converted into the substance of the perfect 

 animal ; the rest is either absorbed by the growing animal, or 

 shrivels up and disappears. 



Below the twelve models in Case 36 may be seen a representation 

 of three stages in the history of the Feather-star {Antedon bifida). 

 The larvse of this Echinoderm are not free, but are attached by a 

 stalk (Fig. 6) ; in the common Feather-star and other Comatulidge 

 the stalk is found during larval stages only ; in others, such as 

 Penfacrinus, it persists throughout life. 



The presence or absence of this stalk has been taken as the first 

 character of importance in the classification of Echinoderma which 

 may be divided into two groups : — 



A. Pelmatozoa,* or Echinoderms provided with a stalk through- 



* From the Greek pelma = a stalk. 



