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 Avill 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 Loven,* we have (Fig. 257) in the radial plate (/?) 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, h, is much retarded. It seems, 

 indeed, not improbable that p, Fig. 257 (Loven, o]). cit.), is one of the 

 series of plates along the middle aboral line of the ray, three of Avhich 

 are shown in Fig. 259. Possibly these plates are homologous with the 

 dorsals of Amphiura and other Ophiurans. Loven'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 1 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 (arf^). 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 like a dorsal, and when grown occupies 

 also the position of both laterals and possibly the ventral. Is it homo- 



* Etudes sur les Echinoidees, Kongl. Svenska Vetenskaps Handlingar, Bandet 

 11, No. 7. The plate p, according to Loven, is a plate of the " systeme periso- 

 matique." In the very young there is a single median series in this species. 



