PEIMAEY LAEV'AL PLATES OF BRACHIATB ECHINODERMS. 25 



differ from the conclusions arrived at by some of my prede- 

 cessors, and in the second place I trust to be able to bring 

 forward some details in the structure of the Asteroidea which, 

 in my opinion, elucidate difficulties that have hitherto been 

 the theme of doubtful contentiou and diversity of opinion. 



In studying the homologies of the primary calcaieous 

 elements of the test, much difficulty and some inaccuracy has 

 hitherto arisen from ignoriug the fact that the corresponding 

 plates in the different orders of Echinoderms do not appear at 

 the same period of growth in the larva of each, or, in other 

 words, that the first appearance of a given series of primary 

 test plates occurs at a different stage of the development of 

 the larva in relation to the other series of plates in the 

 different orders ; e. g. in Ophiurids the radial plates are 

 generally well developed before any trace of the basals appear, 

 whilst in the Asterids the basals have already attained a large 

 size before the primary radials are represented. To this sub- 

 ject I shall have occasion to refer again presently. 



In the diagrams given on Plate I the respective larvae are 

 represented at a stage when all the so-called primary plates 

 are present and consequently when due comparisons can be 

 made between them. I shall now proceed to remark briefly 

 upon a type from each of the orders. 



The Crinoid as the standard of comparison demands our first 

 attention. Fig. 10 represents a diagram of the plates of the 

 young Antedon rosacea at the stage just before separation 

 from the stem takes place. The respective plates are lettered 

 in accordance with the nomenclature proposed by Herbert 

 Carpenter,^ and with which I entirely agree; his deductions 

 being perfectly logical and in my opinion conclusive. The 

 first radials (4, 4) are enlarged and enclose the circle of smaller 

 basal plates (3, 3,) excluding them entirely from touching the 

 equatorial zone. The centro-dorsal plate has been removed 

 and its position is indicated by the space marked 1, which also 

 represents the telescoped stem and radical plate or root- disk, 



^ "Oral and Apical Systems of the Echinoderms," ' Quart. Journ. Micr. 

 Sci.,' vol. xviii. 



