The dorsal primary spines of the heart-urchin, Lovenia elongata (GRAY) were measured and compared with each other. The spines increase in number as well as in size with the growth of the test, in other words, as development proceeds, larger spines make their appearance later on. Two primary spines standing nearly symmetrically with regard to the III-5 plane of symmetry are nearly equal in lengths. When to spines are nearly equal in length, no matter to which side of the body, right or left, they belong, they probably are those which grew almost in the same period.
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