OPHIUROIDEA, or “Brittle-stars.”—These Echinoderms appear to resemble the ordinary Starfish[25]; 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 _Astrophyton_, 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 genus _Hemiaster_ offers an example of an Echinoderm in which the eggs are laid in special pouches; the hinder ambulacra are deepened to form pits, which are guarded by specially elongated spines (see Case 34); in these pits the young pass through all the stages of their development. The minute structure of the spines of Sea-Urchins is illustrated by a series of figures on the wall. The HOLOTHURIOIDEA, or Sea-Cucumbers, form the last order of Echinoderms. Their body, as indicated by their English name, is elongate, subcylindrical, with a more or less flexible integument, according to the extent of the reduction of the calcareous skeleton; the mouth is at one end of the body and surrounded by tentacles, the vent at the opposite end.