14 ON THE EMBRYOLOGY OF ECHINODERMS. 



from the observations of Miiller himself, that I give the accompanying explanation, 

 which seems to bring the figures of Mi'iUer in accordance with what I have observed. 

 The appearance of the teeth, in Miiller's figures, on what seems the abactinal side, is 

 due to compression also. The spines of the young Sea-urchins observed by Miiller have 

 a very uniform appearance ; they are nearly all hexagonal prisms in their earliest stages. 

 The same is the case with our young Sea-urchins, though they lose their embryonic 

 character at an earlier period than is the case in any species observed by Miiller. 



Embrijological classification of EcJiinoids. 



From a careful examination of the different stages through which our young Sea- 

 urchin passes during its development, and after comparing these stages to forms resem- 

 bling them which are now found living in our seas, we obtain valuable hints as to the 

 relative standing of the different families of some Echinoids. We can extend this 

 somewhat to other orders by availing ourselves of the additional observations of Miil- 

 ler, and of a few facts concerning the development of Clypeastroids noticed by Professor 

 Agassiz, which will be gijen here. Professor Agassiz's observations were made upon 

 young specimens of Mellita testudinata Kl., which is so common along the whole of 

 our Southern coast, from Beaufort, N. C, to Texas ; upon the young stages of Echi- 

 narachnius parma Gray, which are found in great quantities in the stomach of our 

 Cods ; and on the young of an Encope, probably Encope Valenciennesii Ag. The ob- 

 servations on Mellita I had occasion to repeat on an allied species, Mellita longijissa 

 Mich., which is exceedingly numerous on the beaches in the neighborhood of Acapulco, 

 Mexico. 



The smallest specimens of Mellita observed, measiiring about one seventh of an inch 

 in diameter, are nearly circular ; they have but a single lunule placed in the posterior 

 interambulacral space ; this is no larger than the prick of a needle. With advancing 

 age, measuring one fifth of an inch, its outline has not varied much ; the lunule has 

 grown larger. When it has attained a diameter of one third of an inch, the outline 

 becomes slightly indented at the place where the two posterior ambulacral lunules will 

 eventually be developed. As the posterior interambulacral lunule is not homologous 

 to the others, but is simply an opening for the anus, we have in the first stages de- 

 scribed a condition which reminds us at once of Echinarachnius. It next loses its 

 circular outline, becoming somewhat eccentrically elliptical, as in Dendraster ; while 

 after the formation of the two posterior lunules it has all the aspect of a Lobopliora. 

 As it increases in size two other indentations are formed, the indication of the position 

 of the two anterior ambulacral lunules. These lunules remain open for a considerable 

 period of the growth of the urchin ; Ave cannot fail to recognize in this state of the 



