METAMORPHOSIS. 



283 



interstitial tissue filled with round cells is formed external to 

 the lateral discs, and with participation of the thickening skin. 

 This tissue becomes the seat of calcareous deposits, and forms 

 the dermal skeleton of the adult Echinoderm (fig. 228 a, b). 

 The canal of the dorsal pore has in the meantime changed its 

 simple form and developed into the circular vessel with diverti- 

 cula, which are destined to become the ambulacral trunks. As 

 development progresses, the young animal appears as a, more or less 

 spherical or pentagonal body, or as a star with short arms, in propor- 



FIG. 228. E [pinna?' a from Trieste forming a stage in the development of the Siar-fisli 

 (St) (after J. Miiller). a, Earlier stage. M, stomach ; A, anus ; V t ambulacral rosette 

 with ciliated tube opening by the dorsal pore. I, Older stage. 



tion as it predominates over the larva. Finally, after the sprouting 

 out of the ambulacral feet, the young Echinoderm becomes separated 

 from the larval body, which not unfrequently remains attached to 

 t';e former, like the remnants of a broken-down framework. The 

 stomach, which is taken into the interior of the body of the 

 Echinoderm, is torn from the oesophagus of the larva (Bipinnaria), 

 and acquires a new resophagus and mouth. The dorsal pore becomes 

 the pore of the madreporic plate. 



The Synaptidce, on the contrary, are formed by the transformation 

 of the entire body of the Auricularia. Five tentacles appear in front 



