MUSEUM OF COMPARATIVE ZOOLOGY. 5 



plates to one another, and to aid in their identifications and homologies. 

 A consideration of the internal organs is a most interesting and neces- 

 sary chapter in a study of the growth of the stellate form of the starfish, 

 but it is one of which little is written in the present paper. Some idea 

 of the origin of organs in immediate connection with the plates is neces- 

 sary, however, to understand the homologies of the calcareous formations 

 with which this paper is specially concerned. 



The development of the brachiolaria of our common Asterias is well 

 known through the researches of A. Agassiz,* and is not here considered. 

 My account opens with a late stage of the brachiolaria, in which certain 

 calcareous nodules, described in the paper mentioned, have already ap- 

 peared, and in which the form of a stellate animal is obscurely marked 

 out. It is intended first to follow the general course of growth of these 

 plates collectively, and later in the paper, the development of individual 

 plates will be taken up one after the other. 



In the starfish body, as is well known, there are two regions, called 

 the actinal and abactinal, the lower and upper, ventral and dorsal, which 

 may be studied. The primary plates in these two hemisomes differ 

 from the very first in number, arrangement, and distribution. No plate 

 is ever formed in the centre of the actinal hemisome comparable with 

 that in the middle of the abactinal, and it would be a task which the 

 author is not called upon to undertake to compare the ten ambulacrals 

 formed on the lower hemisome with the five terminals and five genitals 

 of the abactinal region of the body. 



In the early condition of the plates there is an indication of the disk- 

 like form which the young Asterias has, but it is somewhat masked. 

 If we look at the lower or anal pole of the brachiolaria (PI. I. fig. 1) 

 laterally, and in such a way that the forming plates are on the side 

 turned to the observer, we can see ten small calcifications, an'anged in 

 two U-shaped lines, one within the other. If we so place the brachio- 

 laria that the anal pole is below, or pointing to the lower side of the 

 figure, the madreporic body on the left hand of the observer and the 

 anus of the brachiolaria on his right, we notice the five plates, now in 



* On the Embryology of Asteracanthion heryUrais, Ag., and a Species allied to 

 A. rubens, M. T., Asteracanthion pallidus, Ag. Proc. Aiuei'. Acad. Arts and Sci., 

 VI., 1863. Also separate, 1863. 



Embryology of the Starfisli, published in December, 1864, advance Pt. I., Vol. 

 v., Contrib. Nat. Hist, of U. S., of L. Agrassiz. — The same, reprinted with descrip- 

 tions of the hard parts (calcareous skeleton) of several genera and species of 

 Asteroidea, under the title, " North American Starfishes," Mem. Museum Conip. 

 Zoology, v., No. 9, 1877. 



