32 BULLETIN OF THE 



main difference is, that in the Asterias which I studied the plates on 

 the actinal side of the arm originate on that side, and there is no 

 growth downward from the abactinal surface enclosing the water sys- 

 tem. The ambulacral rafters and interanibulacrals originate as separate 

 calcifications on the actinal side of the arras, while no absorption of 

 plates previously formed was observed. 



There is a close similarity in the early formation of the actinal plates 

 in x\sterias and of those of Asterina, followed b}' Ludsvig, and my in- 

 terpretation of some of the plates of the mouth is in most cases the 

 same as his. 



Ludwig says (p. 49, op. cit.) that he is the first to make known the 

 primary position of the ambulacrals.* The difference in the early form 

 of the first and second pairs of ambulacrals, or those which form the 

 oral ring, is not especially considered by him, and his account does not 

 extend to the growth of the ambulacrals formed subsequently to the 

 oral or first pair. 



It was not possible for me to observe any relationship in the time 

 when the members of the five pairs of ambulacrals form, or their se- 

 quence, as he has done in Asterina, although I have repeatedly found 

 young stai'fishes in which one pair of ambulacrals (oral) smaller than 

 the remaining, or in which one or more members of the five sets were 

 missing. 



The five pairs of plates which Ludwig {op. cit.) letters JA in his ac- 

 count of Asterina are called bv him interambulacrals. Bv this term 

 it is understood that he means what are here called marginals. In the 

 development of these plates Asterias closely resembles Asterina. 



Adoral to these plates lie five other plates, a single plate in each 

 interradius. These arc the first interbrachials, and are regarded as the 

 odontophores of authors. They are called the heart-shaped plates from 

 the shape which they have in the young Asterias. In Asterias the in- 

 terbrachial ends of the oi'al ambulacrals arch over the heart-shaped 

 bodies before the "lateral plates " are developed.t 



* Ludwiii was the first to show how the ambulacrals originate in Asterina. 

 Tlie form and early condition of the oral ambulacrals of an Asterias-like star- 

 fisli with a brachiolaria was given by Krohn in 1853. Thomson [op. cit.) in 

 ISlil figures correctly the first form of the ambulacrals in A. violaceus. It would 

 therefore seem that in genera besides Asterina the subject had already attracted 

 observers. 



t Ludwig was at first of the opinion that these odontophore plates are " Inter- 

 mediiire Skelettplatte," which I interpret to be the same as " orals"; lie later sup- 



