THE WONDERS OF LIFE 



symmetrical larva (which as yet shows no trace of 

 the five-rayed structure). By a very curious modifica- 

 tion the small bilateral astrolarva is transformed into 

 the totally different pentaradial astrozoon, the large 

 sexually mature echinoderm with a pronounced five- 

 rayed structure. (See Art-forms in Nature, plates lo, 

 20, 30, 40, 60, 70, 80, 90, and 95.) It has a most elab- 

 orate organization, with muscles and cuticular skeleton, 

 blood-vessels and water-vessels, etc. A section of the 

 astrozoa — the living crinoidea, or sea-lilies, and the 

 extinct classes of blastoidea (sea-buds), cystoidea (sea- 

 apples), and amphoridea (sea-urns) — grow in stationary 

 fashion at the bottom of the sea. The other four extant 

 classes creep about in the sea — the sea-gherkins (holo- 

 thuria), the star-fish (asteridea and ophoidea), and the 

 sea-urchins (echinidea). Their creeping motion is accom- 

 plished by two kinds of organs — water-feet and skin- 

 muscles. The latter find their support and attachment 

 in solid calcareous needles, which develop from chalky 

 deposits in the corium. As these calcareous needles 

 (which are particularly conspicuous in the sea-urchin) 

 are set movably in special protuberances of the cal- 

 careous plates of the cuticular skeleton, and moved by 

 little muscular needles, the echinoderms walk on them 

 as if they were stilts. Between these, however, a num- 

 ber of water-feet arise from inside — thin tubes like the 

 fingers of a glove, which are filled with water by an in- 

 ternal conduit-system (the so-called ambulacral system) 

 and become stiff. These very extensible ambulacral feet, 

 often provided with a suctorial plate at the closed outer 

 end, serve for creeping, sucking, touching, and grasping. 

 As these distinctive motor organs of the echinoderms 

 — both the ambulacral feet with their complicated water- 

 tubes and the movable needles with their joints and 

 muscles — are found in hundreds, often in thousands, on 

 every individual five-rayed astrozoon, we might say that 



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