140 BULLETIN OF THE 
end of the water-tube is said by him to pass through the tube thus 
formed. 
The significance of the terminal plate has played an important part in 
discussions of Echinoderm morphology, and is by many thought to be the 
same as the ocular plate of the starfish. This opinion seems well sup- 
ported ; but whether the terminal or the radial is the homologue. of the 
ocular of the sea-urchin is open to discussion. 
As to the exact relationship of the terminal plate to other plates of the 
arm, it may be well to inquire whether it cannot be homologized with 
the so-called dorsal plate of the first or second adambulacral plates of the 
actinal region of the disk. That question will be considered, not answered, 
in our discussion of the dorsal plates of the arms. 
When the arm is broken, a new terminal is formed by being pressed 
out by the growth of new plates, just as in the originally formed ter- 
minal plate. In the very instructive figures of the young of Asterias 
glacialis, Linn., by Lovén,* we have (Fig. 257) in the radial plate (p) a 
structure which may be considered the primary radial. If, however, we 
regard p as homologous to the radial plate, rp, of the Amphiura young, 
as its position on the radius would seem to indicate, its time of develop- 
ment, as compared with the interradials, 6, is much retarded. It seems, 
indeed, not improbable that p, Fig. 257 (Lovén, op. cit.), is one of the 
series of plates along the middle aboral line of the ray, three of which 
are shown in Fig. 259. Possibly these plates are homologous with the 
dorsals of Amphiura and other Ophiurans. Lovén’s figures (Figs. 256, 
258) are interesting in another way. In the former of these, which is a 
view from the actinal side, in each interradius, there are two plates which 
in Fig. 258 bear spines. What are these plates? No one seems to 
have asked the question, as it is perhaps thought to be self-evident that 
they are adambulacral. As compared with the young Amphiura they 
have similarities with the second pair of adambulacral (ad?). As in 
Amphiura these plates, although homologous to the lateral plates, are 
somewhat modified, so in the young A. glacialis they are somewhat dif- 
ferent from other adambulacral plates. Among all the plates of the 
arm the terminal offers this peculiarity. It is the only plate which ori- 
ginates on the dorsal side of the water-tube and grows around it to the 
ventral. It therefore originates ike a dorsal, and when grown occupies 
also the position of both laterals and possibly the ventral. Is it homo- 
* Etudes sur les Echinoidées, Kongl. Svenska Vetenskaps Handlingar, Bandet 
11, No. 7. The plate p, according to Lovén, is a plate of the ‘‘ systeme périso- 
matique.” In the very young there is a single median series in this species. 
