METAZOA ECHINODERMA. 171 



The radial shields increase in size. The arms divide 

 many times and the great number of flexible branches 

 intertwine with one another, giving a basket-like appear- 

 ance, and the name of Basket-fish, to the animal (No. 313). 

 These arms are without the dorsal or ventral plates pecul- 

 iar to most Ophiurans, though there are irregular plates 

 which may be vestiges under a thick skin. The arms are 

 without spines. In some species of Astrophyton there 

 are no oral shields, while there may be one madreporic 

 body or five. 



ECHINOIDEA. 



The primitive rocks of the Lower Silurian formation 

 have yielded a primitive sea urchin whose marked sim- 

 plicity of structure offers a sufficient reason for consider- 

 ing it as an ancestral form. This urchin, Bdthriocidaris 

 pahle?ii Schmidt, by name (PI. 314, fig. i, enlarged twice), 

 has a globular corona (popularly called shell) with the 

 mouth in the middle of the lower or abactinal side and 

 the anus opposite. It has a small number of simple 

 spines, a few of which are seen attached in the figure. 

 The spines are only 4 mm. long and are therefore not of 

 disproportionate length. The ambulacra are the first 

 areas to be developed, around the oral disc or peristome 

 (fig. 2) ; they are, therefore, of primary importance, while 

 the interambulacra arise secondarily in the spaces between 

 the ambulacra. Beginning at the center of the actinal 

 area it is seen that there are two complete circles of 

 ambulacral plates extending around the mouth, then 

 comes a circle of ten ambulacral plates and five interam- 

 bulacra 1 plates not wholly seen in fig. 2, i' . The ambu- 

 lacral plates are pierced by two holes which are separated 

 by a partition. It is seen that each ambulacrum origi- 

 nates in two plates, while each interambulacrum arises 

 from one plate. This stage is permanent throughout the 



