61 



likewise subsequently to increase in thickness; as processes issue also from the surfaces of 

 the disc, ramifying themselves in a similar manner. At last the ramification of the calca- 

 reous staves becomes so manifold and dense, that the whole forms a compact calcareous 

 mass, in which however the original reticular or lattice-work formation may easily be ob- 

 served by applying sufficient magnifying power (fig. 23, 24). 



The ambulacral plates, which thus originally, like the adambulacral plates, had the 

 form of thin circular discs, increase relatively very rapidly in volume, and soon acquire 

 (see fig. 14, a) a transverse position on each side of the medial line. Both ends, the exterior 

 as well as the interior, become gradually enlarged, and meet the nearest adjacent ambu- 

 lacral plates belonging to the same side, whereby these plates acquire a somewhat fusiform 

 shape. Between them there will thus remain oval apertures, which are the ambulacral pores, 

 whence the water-feet project. The interior extremity of the ambulacral plates developes 

 itself then more strongly than the exterior, and begins to raise itself up in the interior of 

 the arm to form the dorsal ridge of the arm-vertebrae which is afterwards so prominent. 

 Still however the ambulacral plates of each pair are separated in the medial line by a 

 distinct fissure widened between the vertebrae (see fig. 16, a). The adambulacral plates (ad) 

 grow relatively much more slowly, and remain therefore in respect of size far behind the 

 nearest ambulacral plates; they are a little elongated, and ranged in a somewhat oblique 

 direction in the intervals between the exterior ends of the ambulacral plates, aud soon come 

 in contact with these extremities. 



The further development of the skeleton of the arm consists in each pair of ambu- 

 lacral plates belonging to a vertebra growing together with each other and with the respec- 

 tive adambulacral plates. Afterwards the whole vertebra becomes gradually elongated during 

 the further growth of the arm (fig. 16). In the young Brisinga above described even the 

 interior vertebrae of the arm were as long as they were broad; and in young specimens with 

 a disc-diameter of 10 mm the joints of the arm (see Tab. V, fig. 15) are still quite unusually 

 elongated in comparison with those of full grown individuals, in which, in the whole basal sec- 

 tion, they are scarcely half as long as they are broad. As to the ambulacral skeleton of the 

 disc, it has been already stated that it consists originally (in the earliest stages of develop- 

 ment) only of the interior arm-joints that are connected with each other round a common 

 centre (see Tab. V, fig. 11, 12). Besides the ambulacral and adambulacral plates belonging 

 to these vertebrae, there are still only the so-called parietal plates (p) distinctly developed; 

 these are however yet very small, and appear indeed only to contribute to the formation of 

 the circular border (x) to which the oral membrane is attached. Of the wedge-plate only 

 slight indications are observable (w), and these seem to shew that they ought rather to be 

 reckoned among the calcareous parts of the articular system than as belonging to the ambu- 

 lacral skeleton; although they are subsequently firmly connected with the latter. The same 

 may be said with even greater certainty of the dorsal marginal plates, which are connected 

 with the wedge-plate and which in like manner are, in fully developed specimens, firmly 



