MUSEUM OF COMPARATIVE ZOOLOGY. 139 



developed dorsal shields (primary radials?). Ludwig* regards this inter- 

 pretation as incorrect. A. Agassiz has shown that this plate is forced out 

 by growth of the arm to its distal extremity. Metschnikofl't correctly 

 figures the terminalia, and regards them as the " ersten Anlage der Wir- 

 belstiicke." The true arm-joints or ambulacral plates, which are regarded 

 the same as the " Wirbelstucke," are formed as a rule in pairs, not medi- 

 ally and unpaired. Ludwig J gives an accurate account of the growth of 

 the terminal plates, and shows that they form early in the career of the 

 young Amphiura. He is doubtful whether they originate before or after 

 the radialia, and regards it as probable that they are formed before the ra- 

 dialia or primary radials. Carpenter and Sladen § have discussed the 

 homologies of the terminalia, adding no new facts, but drawing for illus- 

 tration from the excellent paper of Ludwig so far as the development 

 in this genus Amphiura is concerned. Sladen § says : " The primitive 

 structure and mode of formation of the terminal plate is different from 

 that of the first radial." It seems to me that the difference in primitive 

 structure, if any, ought to have been more fully pointed out, and it is 

 doubted whether there is any great difference in these particulars. I 

 find nothing in Ludwig's account to justify the above statement of 

 Sladen, and my own observations show that both the terminal plate 

 and the first radial have many points of resemblance in " primitive struc- 

 ture " and " mode of formation." It is not intended to be denied that 

 the form of the terminals and first radials may differ from the very first, 

 or that they cannot be distinguished one from the other. 



The terminals originate after the primary radials. This statement, 

 hardly in accord with that of Ludwig, is supported by the figures of 

 Krohn, Schultze, and Metschnikoff,t in which the terminals are much 

 smaller than the radials, and in which the indications are that the latter 

 are just forming. My own observations support the statement. I have 

 never, however, recognized a young Amphiura with radials and without 

 terminals. The terminal plate originates on the abactinal side of the 

 water-tube, or " feeler," and by a growth of the arm is pushed out to the 

 very end of the ray (Fig. 12). According to Lyman,|| the terminal plate 

 is a hollow tube ; and according to Ludwig,* it grows from the abactinal 

 side around the feeler on both sides, joining on the actinal side. The 



* Op. ciL, p. 187. t Op. ciL, PI. IV. Fig. 17. 



t Op. cit., pp. 187, 188. 

 § Op. cit., pp. 29, 30. 



II Ophiuridae and Astrophytidse, Old and New, Bull. Mus. Comp. ZoQl., Vol. III. 

 No. 10, p. 258, PI. V. Figs. 1, 2, 3, 4. 



