. Natural history. Zoology. 66o ECI-IINODERMA—CLASS ECHINOIDEA. front margin, while the anus moves downwards to the hinder margin, and eventually comes to lie on the under side of the test. These heart-urchins, as they move along through the sand and mud, scoop it up into their mouths, and pass it through the gut, extracting from it on its passage such nutriment as the minute organisms in it can afford. Urchins of this type have short delicate spines, as shown on the left side of Fig. 8. They move almost entirely by stretching out their long tube-feet. It is, perhaps, the hair-like spines that
. Natural history. Zoology. 66o ECI-IINODERMA—CLASS ECHINOIDEA. front margin, while the anus moves downwards to the hinder margin, and eventually comes to lie on the under side of the test. These heart-urchins, as they move along through the sand and mud, scoop it up into their mouths, and pass it through the gut, extracting from it on its passage such nutriment as the minute organisms in it can afford. Urchins of this type have short delicate spines, as shown on the left side of Fig. 8. They move almost entirely by stretching out their long tube-feet. It is, perhaps, the hair-like spines that have caused the urchin here iigured to be popularly known as Mermaid's head, Child's head urchin, and Hair sea-egg. The sea-urchins, like the star-fish, have pedicellarise. In the regular urchin locomotion is chiefly effected by means of the spines, which are used like crutches; they can also be used like chop-sticks for prehension. The spines also serve as organs of protection, but their efficacy varies much in different forms. In Diadema setostim the spines reach a length of 10 in., and are so fine that one is pricked by them before one can see them. A few urchins, such as Asthenosoma sirens, have poison-glands attached to their spines. On the other hand, the large spines of the Piper appear to be very small protection against its natural enemies, fish and star-fish; it is only the small spines that have any defensive value, and they are placed for this purpose near the main openings and organs of the body. Some sea-urchins cover themselves with dead shells, sea-weed, and similar objects, which they hold on by their tube-feet and so move about unobserved. Other sea-urchins do not move about but live in holes in the rocks, which in some cases they can be proved to have bored for themselves. The sea-urchins here figured have a rigid test; but there are others in which the plates are not so closely joined together, and the test is flexible. Fossil examples of such leathery ur
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