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to the vertebral column of a vertebrated animal; a resemblance which is certainly more 

 remarkable in the present Asteride than in any other (comp. fig. 15. 1G&17). These verte- 

 brae together form the boundary of the wide and deep ambulacral furrow which runs along 

 the ventral side of the arms, and in the bottom of which appear the holes or ambulacral 

 pores situated in pairs and indicating the attachment of the water-feet (see fig. 16). Each 

 vertebra (Tab. Y, fig. 3— 6) is composed of 2 ambulacral plates (aa) firmly connected with each 

 other by a suture in the middle, forming the upper convexity of the ventral furrow, and of 2 thick 

 cylindrical adambulacral plates (ad ad) which form the borders of the furrow on each side. In 

 addition to these there are on the 2 interior vertebrse of the arms (fig. 15—17) a pair of peculiar 

 plates (c c) situated on each side above the adambulacral plates, and which, according to 

 their place, must be considered as answering to the dorsal marginal plates in other star- 

 fishes. The connexion between the individual vertebrae is effected by means of smooth 

 articulating surfaces which in the circumference are partially attached to each other by 

 elastic muscular bands. Of these joint-surfaces there may be distinguished (see fig. 19—20) 

 at each end of the vertebra, 4; namely 2 central, situated close together (a a) whereby the 

 2 dorsal parts of the ambulacral plates which meet in the middle are connected with those 

 of the preceding and of the following vertebra, and 2 lateral (b b) whereby the adambu- 

 lacral plates of 2 vertebrae are connected with each other. This last connexion seems to be 

 the most movable, and is effected with considerably inclined joint-surfaces. On the adoral side 

 of the interior vertebra of the arm, there is moreover a 3"^ pair of articulating surfaces 

 (c c) connecting the dorsal marginal plates with the skeleton of the disc. 



If we regard the skeleton of the arm in its entirety, it exhibits on the upper part 

 (see fig. 15) along the middle, a rather high ridge or keel with a deep medial furrow. This 

 ridge represents the central part of the vertebrae of the arm, and is formed by the dorsal 

 part of the 2 contiguous ambulacral plates. On each side of this ridge the skeleton of the 

 arm is perforated by a series of circular apertures, which on the ventral side (see fig. 16) 

 appear at the bottom of the ambulacral furrow as the ambulacral pores, situated close 

 together in pairs, for the water-feet. These apertures are separated from each other by nar- 

 row partitions inclining downwards and outwards from the dorsal ridge, with their flat en- 

 larged extremities resting on, and attached to the thickened margin, formed by the adambulacral 

 plates, which makes the lateral boundary of the ambulacral furrow. At the bottom of this 

 furrow there is along the middle a narrow and rather deep furrow or semi-canal, whicli in 

 some places is covered with connecting ligaments. It is destined to contain the radial am- 

 bulacral vessel which extends through the wliole length of the arm, and from which the 

 water-feet with their ampollac are supplied with water. 



The limits of the individual vertebrae are, both on the dorsal and on the ventral 

 side, very distinct, and even externally indicated by evident intervals or slits covered with 

 elastic fibres, which allow the whole arm to be bent or extended. On the dorsal ridge these 

 slits have (see fig. 15) an inward and somewhat converging direction, and are especially very 



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