xvi ECHINODEKMATA 499 



has been given by Ludwig (1881), but Fewkes (1887) has also 

 published a paper on the subject. We find on the aboral side of the 

 larva a central plate, the so-called dorso-central, which is surrounded 

 by five plates which are interradial in position. One of these plates 

 surrounds the madreporic pore, which is thus at first situated at the 

 edge of the dorsal surface, not on the oral surface, where it is found in 

 the adult. Fewkes terms these plates basals. Alternating with 

 these are found a circle of five so-called radials, not to be confused 

 with the radial shields of the adult, a pair of which occur above the 

 insertion of each arm. Farther out, overlying the tips of the lobes 

 of the hydrocoele, grooved plates, concave below, represent the 

 terminals. These plates protect the terminal tentacle which is formed 

 from the tip of the primary hydrocoele lobe. Later they become 

 converted into cylinders by the meeting of the edges of the groove. 



On the oral side, alternating with the tube feet or secondary lobes 

 of the hydrocoele, spicules of calcareous matter make their appearance. 

 These are Y-shaped, the stem of the Y being directed towards the 

 point of the arm and the fork towards the mouth. The fork is really 

 the beginning of a system of dichotomous branching, by which a 

 mesh work is produced, since the members of further dichotomies unite 

 with one another just as has been described in the formation of an 

 Asteroid plate. A similar process of dichotomy begins later in the 

 end of the stem. 



In this way each Y forms a narrow plate, elongated in the 

 direction of the arm. The proximal ends of the right and left plates 

 unite with one another first, and then the distal ends. In this way 

 the beginning of a vertebra is formed. It is obvious that the two 

 plates correspond to the ambulacral plates of an Asteroid. In the 

 genus Ophiohelus the " vertebrae " consist throughout life of two rods 

 joined at either end with a hole in the middle, just as they are in the 

 young Amphiura squamata. 



Other plates are formed on the sides of the arm, these are 

 lateral plates or adambulacrals, and on the aboral side other plates 

 make their appearance. Between dorso-central and so-called basal, 

 on each interradius, two plates appear. These force the basal round 

 to the ventral side of the young star-fish, and their appearance is 

 therefore correlated with a growth in length of the interradii ; the 

 radii grow more slowly. 



A plate, the so-called underbasal, becomes interposed between the 

 radial and the centro-dorsal, but the radial is not forced far out or on 

 to the ventral surface. On the contrary the greatest growth takes 

 place between the radial and terminal, and is correlated with the 

 growth in length of the arm. The true " radial shields " appear as 

 paired plates between the embryonic radials and the terminals. The 

 first two pairs of ainbulacrals do not unite with one another, but each 

 unites with the first adambulacral or lateral, and this forms half the 

 jaw-frame, the other half being formed by the similar plates in the 

 next ray. 



