Animal life in the sea and on the land . en-eral plan of a sea-urchin. Do not imagine that star-fishesever do turn into sea-urchins. This is merely intended toshow you the similarity of their structure. 5. According to this arrangement the double rows ofperforated plates would represent the middle of the raysof the star-fish where the tube-feet are situ-ated, while the broadbelt of knobs corre-sponds to the strip ofspines on both edgesof the ray. Themouth would be un-derneath, and youwould of course lookfor the five eye-specks on the top where the Fig> of a Sea-urchin. ends of t
Animal life in the sea and on the land . en-eral plan of a sea-urchin. Do not imagine that star-fishesever do turn into sea-urchins. This is merely intended toshow you the similarity of their structure. 5. According to this arrangement the double rows ofperforated plates would represent the middle of the raysof the star-fish where the tube-feet are situ-ated, while the broadbelt of knobs corre-sponds to the strip ofspines on both edgesof the ray. Themouth would be un-derneath, and youwould of course lookfor the five eye-specks on the top where the Fig> of a Sea-urchin. ends of the rays meet. a> m(rath; c, stomach; d, intestine; o, anus; You will also find themadreporic body atthe top, a little on one side. This small sieve, which isso interesting in the star-fishes, performs the same servicefor the urchin, and prevents the entrance of any sand orother solid substance into the five tubes that pass underthe holes in the shell. Through these holes are pro-truded the double rows of tube-feet, just like those we. v, heart; /, madreporic body ; g, main water-tube ; p, tube-feet; z, spines. 66 ANIMAL LIFE IN THE SEA AND ON THE LAND. have studied in the star-fish, and they are worked in thesame manner. It adds much to the beauty and interest ofthe shell to know that these lovely fine dots are openingsthrough which the tube-feet are supplied with water frominside the shell. In the diagram (Fig. 39) the madreporicbody is shown at/, and the tube, g, carries water to supplythe tube-feet and their little water-bags, which are shownat p. Sea-urchins move by means of their tube-feet, whichmay be lengthened so as to extend far beyond the spines. 6. Growth of the Shell.—The shell fits the animal ex-actly in its infancy, and must still serve it in old age, forurchins never cast off their coats as crabs and lobstersdo. Being formed of many small pieces, the shell growsa little all over. Each plate is surrounded by living flesh secretes lime from the sea-
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1887
