114 STARFISH GALLERY. 
each other like vertebrae, the series extending from the mouth to the 
end of the arms. The groove in which the ambulacral feet are 
arranged is uncovered. 
Typical specimens of this Order are exhibited in Cases 2 & 3, in 
which the great variety of form in the genus Asterias and beautiful 
examples of Acanthaster are shown. Cases 6 & 7 contain specimens 
illustrating the curio as habit of self -mutilation possessed by so many 
Echinoderms ; among Star-fishes, and notably in the genus Linckia, 
the single arms separated from the disk are able to develop a fresh 
disk and arms, and so to multiply the species. Cases 9-11 contain 
fine series of Oreaster. 
Ophiuroidea, or " Brittle-stars." — These Echinoderms appear to 
resemble the ordinary Starfish * ; but they differ in having the organs 
of digestion, respiration, and reproduction confined to the disk, the 
arms having merely the function of locomotor organs. The arms 
therefore are more slender and cylindrical in form, and are sharply 
distinct from the disk ; the separate joints consist of two central 
ossicles, which leave only a narrow canal between them, and these are 
covered above, below, and at the sides by specially developed investing 
plates ; the lateral plates bear spines, which are always comparatively 
short and delicate, as compared with the spines found at the sides of 
the arm in starfishes. 
The principal types of this Order are exhibited in Cases 17-22 ; 
the most exquisite of them are the forms whose arms are divided and 
subdivided till they end at last in the finest threads, as in Astro- 
phyton, the so-called Basket-fish or Gorgon's heads. 
Echinoidea, or " Sea-Urchins," are Echinoderms in which the 
rays are not free, as in the Starfishes or Brittle-stars, but unite to 
form a compact, spherical, heart- or disk-shaped test ; this test is 
covered with spines, which may attain to a great length, as is shown 
in the fine example of Diadema saxatile from the Andaman Islands ; 
some of the tests are flexible and very fragile. Owing to the quantity 
of specimens that are sometimes dredged at one spot, the naturalist 
has been able to gain a better idea of the range of variation in the 
species of Echinoderms than in some other divisions of the Animal 
Kingdom ; an instructive series, showing the variations of Echinometra 
lucunter, is shown in Case 28. 
* The Asteroidea or Ophiuroidea may be counted under the name Stelli- 
formia. 
