S LADEN : STRUCTURE OF ASTEROIDEA. 
279 
examining one of the segments on the under side, after the under 
arm-plate (Fig. 2, d.) has been removed, it will be seen that the 
ambulacral tentacles are situated in a pair of quite small pits, ex- 
cavated, as it were, out of the joint itself. The size of these cavities 
limiting, as a matter of necessity, the capability of the retraction 
of the tentacle to such an extent that they can never be entirely 
drawn in. The vessel of the water-vascular system is situated in 
the angle of the inferior notch, and the nervous system is placed 
external to this. (These systems being coloured blue and red re- 
spectively.) The internal axis of each joint is attached to that of 
the neighbouring segment by very largely developed muscular bands. 
It now remains to point out the homologies that exist between 
the structures just passed in review. 
If one of the axial joints from the ray of an Ophiuran be care- 
fully examined, it will be seen that this vertebra-like body is in 
reality formed of two pieces ankylosed together along the median 
line. (The plate marked a in Fig. 2 being one half.) These two 
plates are in fact the greatly developed and modified representatives 
of the ambulacral plates of the Asteroid (Fig. 1, a.). The side arm 
plates (Fig. 2, b.) are the homologues of the adambulacral plates 
(Fig. 1, b.) ] whilst the dorsal or upper arm-plates of the Ophiuroid 
(Fig. 2, c.) stand in the place of the dorsal and lateral plates of the 
Asteroid ( Fig. 1,1, p.). One other plate yet remains to be mentioned 
(Fig. 2, cZ.), and it is perhaps the most interesting of all, because en- 
tirely wanting in the diagram of the starfish. In the latter the 
ambulacral furrow, with its contained water-vascular and nervous 
systems, is quite open and uncovered ; in the Ophiuran, on the other 
hand, it is enclosed entirely by the ventral or under arm-plate 
(Fig. 2, d.). To account for such a discrepency would be a difficult 
task, if light were not thrown upon the subject by the embryology 
of Asteroids; and we owe the discovery to Mr. Alex. Agassiz' f that 
a similar calcareous plating is present along the furrow of a starfish 
when in a very early phase of growth. This formation, however, 
becomes so modi Bed during the process of development, that all 
* Mem. Mus. Comp. Zool. Harvard, vol. t., pt. 1. 
