EGHINODERMA TA . 



473 



external aperture of invagination and the growth of the thickened disc, con- 

 stitute a covering for the disc, called by Metschnikoff the amnion. The 



B A 



FIG. 266. SIDE AND DORSAL VIEW OF A LARVA OF STRONGYLOCENTRUS. 



(From Agassiz.) 



e''. anterolateral arms; 

 of Echinus. 



General reference letters as in figs. 264 and 265. 



v". ciliated epaulettes; w' '. invagination to form the disc 



water-vascular vesicle adjoining this disc grows out into five processes, 

 forming as many tube feet, which cause the surface of the involuted disc to 

 be produced into the same number of processes. The external opening of 

 the invagination of the disc never closes, and after the development of 

 the tube feet begins to widen again, and the amnion to atrophy. Through 

 the opening of the in vagi nation the tube feet now project. The dorsal and 

 right surface of the Pluteus, which extends so as to embrace the opening 

 of the madreporic canal and the anus, forms the abactinal or dorsal surface 

 of the future Echinus (fig. 267, a). 



nated surface which arises on the left side of the Pluteus. 

 surface of the larva (dorsal of permanent Echinus) two pedicellarise appear, 

 and at a later period spines are formed, which are at first arranged in a ring- 

 like form round the edge of the primitively flat test. While these changes 

 are taking place, and the two surfaces of the future Echinus are gradually 

 moulding themselves so as to form what is obviously a young Echinus, the 

 arms of the Pluteus with their contained skeleton have been gradually 

 undergoing atrophy. They become irregular in form, their contained 

 skeleton breaks up into small pieces, and they are gradually absorbed. 



The water-vascular ring is from the first complete, so that, as in Aste- 

 rias, it is perforated through the centre by a new oesophagus. According 



This disc fits on to the actinal invagi- 



On the right 



