li 



PEIMARY LARVAL PLATES OF BEAOHIATE ECHINODEUMS. 41 



cealment, however, is only a question of the special develop- 

 ment of the last-mentioned plates (i. e. modified adambulacrals) 

 in Asterids as compared with Ophiurids^ for when some of the 

 deep-water Starfishes are examined, with which we have 

 recently become acquainted (e.g. species of Porcellanaster, 

 Styracaster, Hyphalaster, and Thoracaster), we find 

 forms in which this difference is by no means so great, a part 

 at least of the odontophore being superficially visible on the 

 actinal surface. 



In conclusion, it may not be out of place to compare very 

 briefly inter se the characters of the earliest stages of the 

 development of the brachiate Echinoderms. Noting, in the 

 first place, for the whole group generally, that a more or less 

 definite centrifugal movement of plates takes place during 

 growth, and that this is specially manifest in the phylogenetic 

 history of the group, we may proceed to indicate two natural 

 sets of plate-developments, viz. one defined by the basal plates 

 — a basi-oral or interradial series ; and another defined by the 

 radial plates — a radio-terminal or radial series ; the proximal 

 or adcentral factors in each series being more or less 

 stationary. 



In the earliest stages of the Criuoid it has already been 

 observed that the basi-oral series primarily constitutes the 

 whole calyx, the radial series being of a later or retarded 

 development. During the process of growth (at least in the 

 form under notice) the radial series develops with dispropor- 

 tionate rapidity, and at a comparatively early and still premature 

 stage comes to predominate over the basal series. 



In the Ophiurid, on the other hand, we are presented with 

 an accelerated radial development, the radials being first- 

 formed, and the basal series not appearing till later. 



In the Asterid we have a retarded radial development as in 

 Crinoids, the basals being first formed, and the radial series 

 being of later growth. 



It is interesting to note that in both the Asterid and the 

 Ophiurid the outer or distal plate of the retarded series appears 

 earlier than the inner or adcentral factor; the oral plates in 



