METAMORPHOSES 



613 



The metamorphoses of the other types of Dipleurula contain 

 no fixed stage. They are what might be called " cataclysmal 

 metamorphoses." That is to say, the outer form and habits 

 of the larva are preserved till the last moment, whilst the 

 organs of the adult are being gradually perfected ; then in an 

 hour or two all trace of larval structures disappears. The 

 Ophiopluteus preserves the larval mouth, round which the 

 hydrocoel grows ; the 

 long lateral ciliated 

 processes are preserved 

 till the animal has 

 attained all the adult 

 characters. Before 

 this, however, it passes 

 through what may be 

 called an " Asteroid " 

 stage in development, 

 in which the ambu- 

 lacral grooves are open. 

 The Echinopluteus loses 

 both larval mouth and 

 anus. It develops the 

 adult organs on the 

 floor of a sac-like in- 

 vagination of the ecto- 

 derm, situated on the 

 left side within a loop 



Of the ciliated band Fig. 290. Brachiolaria fixing itself, x 60. yl^rudi- 

 fFis" 291 B O This ment of the body of the Starfish; jix, fixing 



; " ' ' '" processes. (After Johannes Miiller.) 



invagination becomes 



completely closed. It is termed the " amniotic cavity," and its roof 

 is termed the "amnion." On its floor are developed the primary 

 tentacles, terminating the radial canals, as well as a number of 

 spines. After taking on a creeping life and losing its larval 

 appendages, the young Sea-urchin passes through an " Asteroid " 

 condition, in which the arched dorsal surface, the future periproct, 

 is greater in extent than the ventral, and the radial canals run 

 horizontally out from the water -vascular ring and terminate 

 in free movable podia (Fig. 291, C and J), pod), ending in 

 suckers, in the centre of which are pointed sense-organs. These 



