MUSEUM OF COMPAEATIVE ZOOLOGY. 17 



terminals. This protection to the delicate forming calcareous plates is 

 afforded in later stages in the growth of the starfish arm, even in speci- 

 mens an inch or more in diameter. As the starfish grows older, the 

 terminals lose their prominence. The large spines and pedicellariae 

 which first form on the terminals are specially treated of elsewhere. It 

 may here be said that the spines of the terminals are the first spinfes to 

 form in the Asterid body. The same sequence and predominance are 

 true likewise of the pedicellarise. 



The general form of a terminal of an older starfish (PI. III. fig. 3) is 

 such that it completely covers the tip and a part of the dorsal region of 

 the end of the arm. That part which covers the dorsal tip of the arm 

 is thinner than that upon the sides, and the groove through which the 

 end of the i-adial tube, or the tentacle, passes, is well marked. 



Medial Dorsals of the Arms. — A row of plates along the crest of the 

 abaxial region of the arm of the starfish may be called dorsals or medial 

 dorsals. The median dorsal row of plates does not begin until after the 

 formation of three pairs of ambulacrals, and likewise subsequent to the 

 odontophores, mouth plates, genitals, terminals, and dorsocentral. The 

 starfish has begun to have a pentagonal or stellate form before the first 

 of this series develops. The first (cZ) of these plates to form appears in 

 the medial dorsal line of the radius, in the triangular space between two 

 genitals and the adoral edges of the terminals (PI. IV. fig. J). It ante- 

 dates the adambulacrals and the laterals. The new dorsals (PI. IV. 

 figs. 3, 4) form distally to those which have already appeared. 



The question of what plates in Amphiura the first of the dorsal plates 

 corresponds to will be spoken of later. There are no radials formed be- 

 fore the terminals inside the ring of genitals ; but in other ways, as far 

 as position goes, the oldest of the series of median dorsal plates of the 

 arm corresponds with the first radial of Amphiura. When the arm of 

 the young starfish is broken from its disk, the line of fracture commonly 

 leaves the first dorsal with the arm, not with the disk. The median row 

 (PI. IV. fig. 4) of dorsal plates form in a continuous series on the middle 

 line of the dorsal (abactinal) region of the arm. The newest formed 

 plates, cP, (T, are those outside the plates already formed. They begin 

 as a simple branched calcareous spicule, and broaden into a flat plate. 

 Each median plate bears at first a single spine. The second median 

 dorsal plate forms after the first pair of marginals and the first pair of 

 interambulacrals. The oldest radial has a quadrate form ; the others, 

 when well developed, are triangular, with re-entrant angles, by which 

 are developed lateral rings and a median adaxial extension. 



VOL. XVII. — NO. 1. 2 



