560 



INVEBTEBRA TE MORPHOLOG T. 



terior extremity, forming a postoral ciliated band, the anus lying without 

 the area enclosed by it. In front of the mouth is a trilobed region also 

 surrounded by a band of cilia, the adoral band. In young embryos the 

 adoral and postoral bands are united at the apex, separation only super- 

 vening later. In later stages two additional arms are developed at the 

 sides of the apical lobe, which becomes like the new arms destitute of cilia, 

 and tipped with a group of wartlike elevations. This form of the larva is 

 known as the Brachiolaria. 



A peculiar process, amounting almost to a metamorphosis, occurs during 

 the transformation of the larva into the Starfish. Calcareous plates of the 

 aboral system make their appearance on the dorsal surface of the stomach 



A 



FIG. 256. BIPINNARIA OF Aster acnniliion (after AGASSIZ). 

 an = anus. Jiy = liydrocoel. m = mouth. 



near the posterior end of the body, and oral plates on the ventral surface 

 of the same organ. These two systems, at first rather widely separated, 

 gradually approach each other, and at the same time the internal organ > 

 assume the adult form. Finally the two series of plates unite, enclosing 

 between them the hydroccel, a portion of the digestive tract and of the coe- 

 lom. The original mouth and anus are obliterated, and indeed the anter- 

 ior half of the larva takes no part in the formation of the adult animal, 

 but is gradually absorbed. 



A highly-developed faculty for regeneration occurs in the Asteroidea, 

 the disk being able to regenerate lost arms ; and indeed an arm, with which 

 a small fragment of the disk is in connection, has the power of regenerat- 

 ing all the missing parts. Specimens of the common Starfish Axti'ritiN are 

 in consequence frequently found with one or more of the arms bifid at the 

 tip, or even with an abnormal number of arms. 



