Xo. 1. — Studies from the Neivport Marine Zoological Laboratory. 

 Communicated hy Alexander Agassiz. 



XX. 



Oil the Beveloxunent of the Calcareous Plates of Asterias. 

 By J. Walter Fewkes. 



1. General Observations. 



2. General Changes in External Form brought about by the Growth of the 



Calcareous Plates. 



3. Development of Individual Plates, Eods, Pedicellariae, Spines, and Stone 



Canal. 



4. Comparisons with other Asteroidea. 



5. Comparison of the Plates of Asterias and Amphiura. 



6. Summary. 



7. Explanation of the Plates. 



1. General Observations. 



Asterias, the most common genus of Asteroidea at Newport, in its 

 development passes tliroiigh a braehiolarian stage before it assumes a 

 stellate form. This brachiolaria is one of the most abundant larvse 

 found in our nets in surface fishing at certain times of the yeai'. 

 Although the development of the brachioharia from the e^^ of the 

 starfish has been accurately worked out, and the changes in the exter- 

 nal form of the young Asterias, after it begins to assume a stellate 

 form, have been well described by several naturalists, we are still igno- 

 rant of the mode and place of formation, and the sequence in the devel- 

 opment, of some of the calcareous plates which help to give the stellate 

 form to the young starfish after the absorption of tho brachiolaria. ^Ve 

 need more information as to how the ambulacral plates form, and when 

 they appear, as compared with the dorsoccntral and terminals. We 

 do not know how or when certain plates of the arm appear, and it is 

 desirable to study the character of certain so-called embryonic plates 

 reported to exist on the median line of the actinal side of the arm 

 in the larva. 



VOL. XVTI. — XO. 1. 1 



