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.— BracMolaria fixing itself, x 60. Ast, rudi- 



CFig. 291, B,C). This ""'^°* °^ *'f.,?°'^?.°^ the starfish; Jx, fixing 



^ o J ' / processes. (After Johannes Muller.) 



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 D, pod), ending in 

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



