ECHINODERMA. 
Ill 
are developed and swim about freely ; but in a few (ffemiaster, 
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 larvae of the other Echinoderms pass through a series 
of remarkable changes (Figs. 4 and 5) ; these are illustrated by the 
Fig. 4. 
Fig. 5. 
Developing larvae. 
Pluteus. 
Bipinnaria. 
twelve models of various forms of larvae exhibited in Case 36 ; in 
Case 35 is a set of models showing in detail the changes under- 
gone by a single species (Asterina gibbosa). 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 larvae of this Echinoderm are not free, but are attached by a 
stalk (Fig. 6) ; in the common Feather-star and other Comatulidce 
the stalk is found during larval stages only ; in others, such as 
Pentacrinus, 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. 
