METAMORPHOSES 



613 



The metamorphoses of the other types of Dipleunila contain 

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

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

 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 

 tiir the animal has 

 attained all the adult 

 characters. lief ore 

 this, however, it passes 

 through what may be 

 called an " Asteroid " 

 stage in development, 

 in which the anibu- 

 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. 291,B, C). This 

 invagination becomes 

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

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

 tentacles, terminating the nidial 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, tlie 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), 2^od), ending in 

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



Fl(i. 290.— nrachiolaria fixing itself, x 60. Ast, ru<li- 

 nieut of the body of the Starfisli ; Ji.c, fixing 

 processes. (After Johanues Miiller.) 



