12 BULLETIN OF THE 



Actinal. 



1. Ambulacrals, Ambulacral rafters. 



2. Interambulacrals (Adambulacrals of late authors). 



In addition, tbe following calcifications, appendages to the above 

 plates, are considered": — 



1. Spines. 



2. Pedicellariae. 



3. Stone canal. 



The last is an internal calcification, which is morphologically distinct 

 from the above, and arises in the walls of the water tube. 



It is thought that almost all the larger calcifications of the mature 

 Asterias can be referred to some of the above-mentioned structures. 

 There are, however, spicule-like calcifications, as in the legs, wmich are 

 not considered in this discussion. 



The plates first appear as a small calcareous formation in the midst 

 of cells, which color with reagents more deeply than those of the remain- 

 ing parts of the body in the immediate vicinity. A common form fol- 

 lowing the simplest is the extension of arms — commonly three — 

 which impart to the spicule a trifid shape. The extremity of each 

 branch subdivides, and continues division, anastomosing and joining 

 with other bifurcations from other branches. The thickness of the cal- 

 cification is at first small, being simply that of a spicule-like rod. By 

 the growth and anastomosis of the spicules the plate later assumes the 

 form of an open network. This network, in large plates like the termi- 

 nals and genitals, is open. In other plates, as the median dorsal, the 

 calcification has from the first the form of a disk in which are small 

 pei'f orations. 



The interambulacral plates are never thin, with loose open work, but 

 become compact at the very beginning of their development. It is 

 probable that the open character of the early formed terminals is cor- 

 related with the fact that these plates are formed in the brachiolaria. 

 It is questionable whether such slight rods would be strong enough to 

 preserve the shape of the stellate animal if they occupied the position of 

 ambulacrals, laterals, or dorsals, and were formed at the same time. 

 These latter plates are more compact from the beginning, for obvious 

 reasons. When the brachiolaria is absorbed, and the terminal comes to 

 be pushed out in an exposed position, its form and compact calcification 

 is such as successfully to resist any injury which its exposure might bring. 

 It not only is strong enough for its own safety, but it serves as an effect- 

 ual shield for the newly formed ambulacral, interambulacral, dorsal, and 



