IX 



PHYLUM ECHINODERMATA 



283 



a new mouth and oesophagus (Fig. 320, B, oss), the larval mouth and 

 oesophagus becoming abolished during the metamorphosis. The 

 new mouth is formed in the centre of the hydroccele (ring- vessel). 

 From the stomach, diverticula grow out radially into the developing 



FIG. 323. Views of the larva of Asterina gibbosa in the course of metamorphosis. A, larva 

 of eight days, from the right ; B, left, and C, right view of the larva of nine days ; 1-5, 

 lobes of hydroccele ; I-V, rudiments of arms. (From MacBride, after Ludwig.) 



arms to give rise to the cseca ; 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. 324). In the middle of the abactinal 

 surface is a single 

 central plate (dors). 

 Around this are 

 five basals (bas) 

 one of which be- 

 comes merged into 

 the madreporite. 

 External to these, 

 five radials (rad) 

 appear somewhat 

 later. At the end 

 of each develop- 

 ing arm is a single 

 terminal or ocular 

 plate (term), which 

 is carried outwards 

 as the ambulacral 

 and adambulacral W ^r/w< 



Ossicles of the arm FIQ - 324. Diagram showing the_relations of the chief plates of the 



are developed, 



supporting the 



corresponding eye and tentacle. A ring of secondary radials or 



infra-basals (sec. rad) is developed between the radials and the 



VOL. I. CO 



apical system in the young Starfish, an. anus ; has. basals ; 

 dors, central ; madr. madreporite ; rad. radials ; sec. rad. 

 secondary radials (infra-basals) ; term, terminal. 



