22 BULLETIN OF THE 



lengths parallel with the median water tube, they are now at right 

 angles to its course. The median tube has not yet extended to the 

 extremity of the edge of the terminal, but has formed two and three 

 pairs of lateral branches, — the first formation of the legs of the star- 

 fish. The ten oral ambulacral plates or rods, am, form a pentagonal 

 network, not yet united, but already in the approximate position which 

 it occupies in the adult (PI. II. fig. 3). 



The plates are crescentic, with convexity pointing outward, perforated, 

 closely approximating near the middle line of the radius, and more dis- 

 tantly separated in the interbrachial regions of the starfish. As far as 

 their general appearance is concerned, they resemble incipient ambula- 

 cral plates of later stages of Asterias, as in former conditions they 

 resembled those of Amphiura. 



In an older condition (PI. II. fig. 3) of the oral ambulacral plates, 

 the interbrachial ends grow together, and at the same time become very 

 much more thickened in the interbrachial region. The ten oral ambula- 

 cral plates, am, now form a pentagonal ring about the mouth opening. 

 In this stage the rudiments of two pairs of ambulacral rafters, am 1 , 

 have likewise appeared. 



The outlines of the single member of the oral ambulacral ring of 

 plates at present are as follows. Each oral ambulacral plate has the 

 form of an elongated bar, enlarged at either end. The length of the 

 bar is at right angles to the line of the radius of the arm. On the ab- 

 oral side it is deeply concave, while on the adoral it is straight, slightly 

 curved. The radial extremity is bifid, divided into an upper and lower 

 branch (PI. III. fig. 1). The interradial extremity is enlarged into a 

 massive thickening, forming a club-shaped body whose aboral broad end 

 abuts the lateral wall of the terminal. The mass of the thickened part 

 of the oral interbrachial plate is on its actinal side, while on the abactinal 

 side it is concave, in which concavity fits a heart-shaped plate, ib, later 

 described as the odontophore. This thickening in the interbrachial 

 region of the oral ambulacrals corresponds with the interambulacrals, 

 and these plates represent interambulacrals of the oral ambulacral 

 plates, although they do not seem to be formed as separate calcifica- 

 tions. In early conditions no spines are found on the oral plates or 

 bars. No spines were ever detected in the ambulacral region of these 

 bars, although in older conditions of that part of the oral plates which 

 lies in the interradii spines were found, as in the other interambulacrals. 



The subsequent growth of the interbrachial ends of the oral ambula- 

 crals is as follows. They grow at the expense of the ambulacral orals, 



