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connecting themselves immovably by suture with the adjacent vertebrae. To their exterior 

 side are attached the radial septa, which in a fan-like arrangement divide the coeloma of 

 the disc; and in these septa there are aften developed peculiar calcareous plates, the inner- 

 most of which are articulated with the wedge-plate. In one of the radial septa which 

 include the stone-canal and the so-called heart, this calcareous plate is double; and its 

 homology with the first pair of dorsal marginal plates in the Brisinga seems thereby to be- 

 come highly probable. The so-called parietal plates, which in the Brisinga contribute 

 essentially to form the interior wall of the oral ring, are also found again in the Solaster 

 and other star-fishes. They are however here of much slighter development, and separated 

 from each other by a considerable interval; on the interior side they form a horisontal fur- 

 row for the circular ambulacral vessel, limited below by a projecting sharp edge, which 

 completely corresponds to the circular rim projecting inwards on the oral ring in the Bri- 

 singa. While in the Brisinga it goes uninterruptedly round the whole interior of the oral 

 ring, in the Solaster and other star-fishes it is interrupted at each vertebra in two places, 

 where the circular ambulacral vessel is only circumcluded by ligaments. In fig. 9 & 10, 

 which represent corresponding parts of the ambulacral skeleton seen from above in the Bri- 

 singa and Solaster, the corresponding calcareous plates are indicated by the same letters, 

 in order that their complete resemblance may be more easily observed. 



If we now turn the ambulacral skeleton, and view it from the ventral side (fig. 2 & 8), 

 it will likewise be at once evident that in both forms there are everywhere found comple- 

 tely homologous parts. The deep ambulacral furrows, at the bottom of which the median 

 longitudinal furrow for the radiary water-vessels, and the holes placed in pairs for the water- 

 feet are seen, are bounded in both on each side by a row of plates (adambulacral plates), 

 which are connected with each other by elastic muscular bands, and to which the so-called 

 furrow-spines are attached; the innermost of these plates being, in both, united immovably 

 by suture with its neighbors into one continuous piece projecting under the oral ring. 

 To this piece are attached the so-called oral spines directed in fan-like arrangement to- 

 wards the mouth. That there are apparently in the Brisinga no oral angles to be found, is 

 a consequence of the slighter development of the piece, in connexion with the greater 

 breadth and more solid composition of the oral ring. It will be evident from what has 

 previously been written, that if we imagine the basis of the arms in the Brisinga connected 

 to a certain extent above aud below by skin, we shall have a tolerably normally developed 

 star-fish; just as, if we imagine in a Solaster the incisions between the arms continued up 

 to the last ambulacral vertebra but one, we shall obtain a form pretty nearly corresponding 

 to the Brisinga. That the coeloma of the disc in the Brisinga is not continued beyond the 

 oral ring or the innermost contiguous vertebra, and that the dorsal skin attaches itself im- 

 mediately in the periphery of the oral ring, is again a natural consequence of the reduction 

 of the skeleton of the disc. The interior organs enclosed in the cavity of the disc, the 



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