16 EMBRYOLOGY OF THE STARFISH. Part I. 



and six days old ; compare PL II. Fig. 19 with PL II. Figs. 25 and 27 seen from 

 opposite sides. The great elongation of the oral extremity and the marked separation 

 made by the opening of the mouth between the anal and oral plastrons, cannot fail 

 to be noticed. 



Comparison of Larvae of Asteracanthion pallidas and A. berylimis. — Up to this time, all 

 the larvae described were raised by artificial fecundation from eggs taken out of 

 the ovaries of Asteracanthion berylinus Ag. At the time when I first discovered 

 the larvas of our Starfishes, I immediately examined the ovaries of our two most 

 common species, the A. berylinus Ag. and A. pallidus Ag. I found that the eggs 

 of the former were not sufficiently advanced to be fecundated, while those of the 

 'second species (A. pallidus) had all escaped. I am, therefore, positively certain that 

 all the larvae I am about to describe, belong to the second species, as they were 

 all found swimming about, previous to the time of spawning of the A. berylinus. 

 As the interval between the time of spawning of these two species is not less 

 than three weeks, I had been able, during this period, to make a general sketch 

 of the whole development, from the youngest larva found (PL III. Fig. 1), to the 

 time when the Starfish is formed, before beginning the artificial fecundation of the 

 species just described, the A. berylinus Ag. 



I thus obtained a general knowledge of the changes these larvae undergo, and 

 was enabled, when making the artificial fecundation, to pay special attention to 

 the development of those parts, the origin of which was not easily traced in 

 older larvae. I was able in this way to carry on, at the same time, the compara- 

 tive study of the development of two closely-allied species, belonging, undoubtedly, 

 to one and the same genus, and to see how far differences could already be noticed 

 in their early stages of growth ; a glance at the figures of the young of one 

 species (A. pallidus Ag.) on Plate III., compared with the figures of Plate II. of the 

 second species (A. berylinus Ag.), will show how far the development of allied species 

 diverges. What is particularly characteristic, is the fact that specific differences make 

 their appearance so early. Soon after it became evident that the embryos we were 

 studying belonged to Echinoderms, it was apparent that they were different species. 

 The order of appearance of the characters of the classes, the orders, the families, 

 and genera, is one of the greatest importance in a zoological point of view ; and 

 we owe to Professor Agassiz to have pointed out, that the characters which make 

 their appearance first, are by no means those which have been usually taken for 

 granted ; in the present case, we do not find it possible to discern the class, the 

 ordinal the family, the generic, and the specific characters, in the order in which 

 they are here mentioned. The specific characters are early stamped upon the 

 embryo, and did we but know how to recognize individual differences among the 

 lower animals, as well as we can already in some of the Fishes, we might find that 



