HARDWICKE'S SCIENCE-GOSSIP. 



169 



GRAPHIC MICROSCOPY. 



By E. T. DRAPER. 



No. XX. — Small Brittle Star-fish. 



s-fi' HE illustration repre- 

 sents the ventral 

 surface of the disk 

 of Ophiocoma neg- 

 lecta (the small, 

 or grey brittle star- 

 fish) of the family 

 Ophiuridae, and also 

 exhibits the attach- 

 ments of the five 

 long spinous radia- 

 ting arms ; the ob- 

 ject is sufficiently 

 minute to require, 

 for the exposition 

 of its beauty and 

 symmetrical struc- 

 ture, a magnifica- 

 tion of thirty or 

 more diameters. 

 Specimens in age, and development, necessarily vary 

 in size, but in this particular species, the average 

 diameter of the disk is about one-sixth of an inch, 

 and the rays, or radiating arms, are of such dispro- 

 portionate length, as to maintain and justify the term 

 Ophiuridae (serpent tailed) ; these elongated arms, 

 unlike those of the true star-fishes, have no ambulacral 

 tentacles, or processes. 



In Ophiocoma the length and extraordinary 

 flexibility and adhesive power of the arms are aided 

 by smaller spinous processes, affording the capability 

 of very active powers of locomotion and prehension ; 

 this characteristic is peculiar to the species ; having 

 no perfect sucker tubes, aiding deliberate progression, 

 as seen in the Echini, and true star- fishes, they 

 curiously exhibit far greater activity, the jointed arms 

 possessing a quivering, jerky movement ; undeveloped 

 membranous tentacles are indicated, but never reach- 

 ing the steady crawling capability of similar organs, 

 found in other Echinoderms. 



Generic character depends on configuration of the 

 rays (always five), and on the form and specific 

 No. 248. — August 1885. 



distinctions disclosed in the arrangement of the many 

 plates and scales. 



Under magnification, disks of the Ophiocomre 

 reveal great elegance ; from a point in the centre of 

 the dorsal side, opposite to the position seen in the 

 illustration, is a series of radiating imbricated scales 

 of uniform size, overlapping each other, turning over 

 the circumference of the disk, and eventually reach- 

 ing the quinary plates, forming the mouth and bases 

 of the rays ; the surfaces of all the parts appear 

 smooth, but under high magnification disclose areola/ 

 markings. The central disk is beautifully patterned, 

 and the various pieces, with care, are capable of 

 separation. The mouth is in the centre, leading by a 

 short gullet to a digestive sac ; this one aperture serves 

 for the reception of food, and the expulsion of 

 unabsorbed portions, the aliment consisting of decayed 

 animal substances. In a living condition, surrounding 

 the borders of the mouth, are seen a series of very 

 minute tentaculae, as it is known these apparently 

 helpless creatures are notoriously ravenous. It might 

 be assumed that, beyond the power of ingurgitation r 

 they possessed some enablement of mastication, or at 

 least prehension ; this may be detected ! If one of 

 the five pieces, involving the aperture of the mouth, 

 be laid open, and the under surface examined with a 

 high power, it will be found beset with minute sharp 

 pointed recurved spines, which, if not teeth, evidently 

 have a file-like clutching action ; a perfect disk 

 arranged with one of these quincunx parts folded 

 back, is a striking microscopical exhibit, showing the 

 points referred to, also the cavity of the stomach 

 beneath. 



The rays, or arms, in proportion to the disk, are 

 comparatively long, excessively friable and brittle 

 in the living condition, falling to pieces under the 

 slightest shock, or even touch, rendering it somewhat 

 difficult to capture perfect specimens. These fragile 

 calcareous rays are necessarily permeated by organs 

 of motion, secretion, and sensation ; externally 

 appearing like curved-rounded conical boxes, fitting 

 into each other, each plate or cup edged with 



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