28 BULLETIN OF THE 



in them. In one or two instances, liowever, there are differences, either 

 of observation or of interpretation. It will perhaps be profitable, before 

 ■we can discuss the relationship of Asterias with Ophiurans, that these 

 differences and concordances among Asteroidea be considered. The 

 subject deals with calcareous plates only. 



The most important observations of the way in which the plates of 

 Asteroids develop are those of Krohn, Thomson, A. Agassiz on Asteracan- 

 thion, Ludwig on Asterina, and Lovdn on Asterias glacialis. The way in 

 ■which the plates of Asterina develop is as well known as that of any 

 other Asteroid, if not better. As this development of Asterina pertains to 

 a starfish without a nomadic brachiolaria, and as Asterias has an indirect 

 development with nomadic brachiolaria, it is interesting to compare the 

 formation of the plates in the two types, and to note the differences 

 which occur. "Whatever the character of the metamorphosis of a star- 

 fish may be, — whether it has a nomadic brachiolaria, as Asterias, or 

 carries the young in brood-sacs, as in Pteraster, — it would appear that 

 the sequence of the growth of calcareous plates is little afi'ected by it. 

 How much the abbreviation in early development afi"ects the sequence 

 in the growth of plates is yet to be proved, and a complete series of 

 the young Asterias to compare with Asterina may give us valuable 

 information on this point. The figures of Asterina by Ludwig, and 

 those of Asterias by Agassiz, Krohn, Thomson,* and Loven,t in a way 

 supplement each other, yet much still remains to be done on late 

 stages of both genera.;}: 



For a comparison of the way in which the plates of the abactinal 

 hemisome of the body of Asterias develop with those of other Asteroids, 

 I have little to add to what is known as far as the dorsocentral is con- 

 cerned. The various authors who have written on this subject do not 

 emphasize the fact that it is formed after the terminals and genitals, or 



* Krohn and Thomson figure and describe isolated stages of growth. Agassiz 

 considers the whole subject of the development. 



t Loven figures only later stages with stellate form. 



X It would appear from the relative time and sequence of the appearance of 

 plates in related genera of Ophiurans being very diSerent, that it is not safe to 

 rely upon a similarity in time when calcifications appear in the comparisons of 

 homologous plates. Other naturalists have already commented on this fact. A 

 diversity in the time of the appearance of homologous plates in related species 

 seems to me paralleled in the fact that m two Asterids once thought to be generi- 

 eally the same, and even now, if their adult features alone are examined, regarded 

 as generically identical, one, A. tenera, has no nomadic brachiolaria, and the other, 

 A. herylinus, has such an elaborate 'metamorphosis with this stage. 



