362 



/OOLOGY 



SECT. 



to the condition of the five-rayed starfish (Fig. 291) is effected by the 

 abortion of the larval organ, the further development of the arms 

 and tube-feet, and certain changes which take place in the internal 

 organs. Of these, one of the most important is the formation of a 

 new mouth and oesophagus (Fig. 289, B, ces), the larval mouth and 

 oesophagus becoming abolished during the metamorphosis. Round 

 this new mouth grows the ring- vessel of the ambulacral system. 

 From the stomach, diverticula grow out radially into the developing 

 arms to give rise to the caeca; and later the permanent anal 

 opening is formed on the dorsal surface. 



When the first ossicles are definitely formed they present the 

 following arrangement (Fig. 292). In the middle of the abactinal 



. 2l>2. Diagram showing the relations of the chief plates of the apical system in the young 

 Starfish, an. anus ; bas. basals ; (?o/'s.=dorso-central ; intu/.r madreporite; rt(. nulials ; .$<.<:. 

 mil. secondary radials. 



surface is a single dorso-central plate (dors). Around this are five 

 basals (bas), one of which becomes converted into the 

 madreporite. External to these, five radials (rad.*) appear some- 

 what later. At the end of each developing arm is a single 

 terminal or ocular plate (term), which is carried outwards 

 as the ambulacral and adambulacral ossicles .of the arm are 

 developed, supporting the corresponding eye and tentacle. A ring 

 of secondary radials (sec. rad) is developed between the radials and 

 the dorso-central. In the adult, by the intercalary development of 

 numerous additional ossicles, these primary plates of the apical 

 system, as it is termed, lose their original arrangement, and become, 

 with the exception of the madreporite and the terminal plates, no 



