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endecacnemos as in the present species. That it has not been noticed before in the Bri- 

 singa, is owing to the extreme point of the arm being so easily broken off when the animal 

 is taken. In a perfect arm the organ is always present, and easily distinguishable; the 

 skeleton of the arm forming here a peculiar enlargement, covered with spines, and in the 

 form of a somewhat bent plate, which is destined precisely to protect the organ in question. 

 It is well known that in many star-fish there is at this place a very conspicuous red pig- 

 mentary spot, in which there has been noticed a peculiar structure of the extremities of the 

 nerves completely answering to the so-called chrystal cones in the compound eyes of Articu- 

 lata, for which reason these pigmentary spots have been confidently designated as real appa- 

 ratus of vision. It was therefore reasonable to anticipate that we should find the organ 

 under consideration in the Brisinga likewise adapted to serve in some manner as an organ 

 of sight. Closer inspection has however not confirmed this presumption. The 2 essential 

 parts which characterise the apparatus of sight in other AsteridaB, namely pigment and 

 chrystal cones, are wanting. But it corresponds in its structure more with an other far 

 less conspicuous part, situated close above the organ of sight, in the form of a simple ten- 

 tacle, which in appearance is very little different from one of the ordinary water-feet, 

 excepting that it is unpaired. In the Brisinga the organ in question (see Tab. II, fig. 5, 6 c, 

 fig. 7 a) has the form of a soft cylindrical plug evenly rounded at the extremity, and of 

 relatively considerable size, many times larger than the nearest very rudimentary water-feet; 

 like these it is directed downwards; and it is sheltered as under a vaulted arch, by the be- 

 fore mentioned peculiar calcareous plate (see fig. 5) which is thickly covered with pedicel- 

 lariax In its structure it seems on the whole to correspond pretty nearly with the water- 

 feet. Like them it is hollow in the interior, and is filled with water from the ambulacral 

 vessel. Its walls are rather thick, but seem to want the layers of muscle peculiar to the 

 water-feet. At the extremity, it is surrounded by a thin transparent membrane a little raised 

 from the proper wall, and here, a fine radial striation is observed. The structure of this 

 organ is, as may be seen, very simple, and it might easily be taken for an unpaired water- 

 foot, simply closing at this place the double row of these organs which runs along the ven- 

 tral furrows of the arms, if its unusual size, its peculiar protection, and the evident enlarge- 

 ment of the ambulacral nerve at its base, did not indicate an entirely special function. It is 

 for this reason that I must presume it to be an organ of sense. But what sort of sensation 

 is transmitted through it, is difficult to say. It can scarcely be a pure and simple feeler, a 

 mere tentacle. In this capacity the numerous water-feet far-reaching and movable in all 

 directions might naturally be presumed to suffice; while on the other hand the isolated 

 position, slight mobility, and peculiar protection of the organ here considered, would evi- 

 dently make it less well adapted for such a function. Probably the animal receives through 

 it a more specific conception of its surroundings than it receives by means of immediate 

 contact; and we might perhaps rather be justified in regarding it as a sort of organ of smell 

 analogous to the so-called olfactory papillae in the Arthropods. 



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