. Harper's young people . le with itsmouth down, and bend its rays backward until they meettogether on top, and the edges of the rays touch eachother, we should have the general plan of a not imagine that star-fish ever do turn into sea-urchins. I merely wish to show you the similarity oftheir structure. According to this arrangement the double rows of per-forated plates would represent the middle of the rays ofthe star-fish where the tube-feet are situated, while thebroad belt of knobs corresponds with the strip of spines onboth edges of the ray. The mouth would be underneath,an
. Harper's young people . le with itsmouth down, and bend its rays backward until they meettogether on top, and the edges of the rays touch eachother, we should have the general plan of a not imagine that star-fish ever do turn into sea-urchins. I merely wish to show you the similarity oftheir structure. According to this arrangement the double rows of per-forated plates would represent the middle of the rays ofthe star-fish where the tube-feet are situated, while thebroad belt of knobs corresponds with the strip of spines onboth edges of the ray. The mouth would be underneath,and you would of course look for the five eyespecks onthe top where the ends of the rays meet. You will alsofind the madreporic body at the top, a little 011 one small sieve, which is so interesting in the star-fish,performs the same service for the urchin, and preventsthe entrance of any sand or other solid substance into thefive tubes that pass under those holes in the shell. 452 HARPERS YOUNG PEOPLE. VOLUME Fig. 1.—UNDER SURFACE OF A SEA-URCHIN, SHOWINGKows OF SUCKERS AMONG THE SPINES. Through theseholes are pro-truded the dou-ble rows oftube-feet, justlike those wehave studiedin the star-fishand worked inthe same man-ner. It addsmuch to thebeauty and in-terest of theshell to knowthat these love-ly fine dotsare openingsthrough which the tube - feet are supplied with water from inside the shell. In the dia-gram (Fig. 4) the madreporic body is shown at /, and thetube-feet with their little water-bags at p. Sea-urchinsmove by means of their tube-feet, which may be lengthen-ed so as to extend far beyond the spines. The shell fits the animal exactly in its infancy, and muststill serve it in old age, for urchins never cast off theircoats as crabsor lobsters form-ed of manysmall pieces,it grows a lit-tle all over, as
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Keywords: ., bookcentury1800, bookdecade1870, bookpublishernewyo, bookyear1879
