ECHINODERMATA 



443 



In the metamorphosis of the Spatangoid Pliiteus into the sea-urchin 

 the invagination is said by Metschnikoff to close. At the bottom of it 

 the earliest fundament of the sea-urchin then makes its appearance. 

 Furthermore the "amnion" is said to become detached from the larval 

 skin. In the protrusion of the feet the amnion, as well as part of the 

 larval skin, would therefore have to be broken through. 



Crinoidea. — We left the larva of Antedon rosacea at a 

 stage in which the nearly ovate form exhibited slight curv- 

 ing toward the ventral side. The further development is 

 characterized by the fact that the larva abandons its free 

 life and grows into an attached stalked form. It therefore 

 passes through a stage in which it resembles a stalked 

 Crinoid. This is known as the pentacrinoid stage. Traces 



Fig. 216. — Young sea-urchin {Arhacia pustulosa) with degenerated Pluteus arms 

 attached (after Joh. Müllbh). /, feet; P, pedicellarise; St, spines. 



of this stage are already shown in the free-swimming larva 

 through the fundaments of the skeleton, which make their 

 appearance in the niesenchymatous tissue of the larva. 

 They are first seen as small granules, which, however, soon 

 enlarge into triradial and quadriradial forms, and finally 

 become fenestrated plates (Fig. 217). Two rows of five 

 plates each can be distinguished, — the oralia and basalia con- 

 stituting the calyx, — and a piece lying below these, the future 

 terminal plate of the stem (Figs. 217, 221, and 22^, p. 450). 

 According to Bury, it is this plate of the skeleton which fir.st 

 makes its appearance deep in the body of the larva. Inas- 



