LIMITATION OF FAMILIES 



119 



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specially hard conditions, and are found amongst creatures that live 

 in very cold water, as in the polar regions, or in fresh water, where 

 the strong currents and rapid changes of temperature are hostile 

 to the feeble young, or in the strenuous and, storm-tossed life of the 

 shore. It may well be that of the multitude of young produced, 

 only those that accidentally remained with their parents survived, 

 and that afterwards the total number was gradually reduced and 

 the arrangements for retaining the young with their parents made 

 more perfect. In most of the sea-urchins enormous numbers of 

 eggs are discharged every season ; the number in the common edible 

 form sold in the markets of the 

 south of France and Italy has 

 been calculated at twenty millions. 

 These eggs are fertilised in the 

 sea and the young embryos drift 

 without any help or protection 

 from their parents. In some of 

 the urchins from the Antarctic 

 seas, Sir Wyville Thomson found 

 that there were little shallow 

 pouches on the outside of the 

 shell or test of the female, and 

 that the spines bordering these 

 were long and curved over to 

 form basket-work lids. The eggs, 

 comparatively few in number, were passed into these pouches and 

 there developed directly into small urchins, which thus enjoyed 

 the security and protection given by the mother until they had 

 reached a considerable size (Fig. 26). So also the most common 

 sea-cucumbers produce eggs in enormous numbers, and these 

 develop into unprotected embryos in the water. In a sea- 

 cucumber from the Falkland Islands the two rows of tube-feet 

 (the bivium) along the dorsal surface are rudimentary, and not 

 used for locomotion in the females ; along these a dozen or so 

 of little animals are attached, looking like a row of yellow plums, 

 and remain there until they are nearly fully grown and able 

 to live the independent lives of the adult. Although in most 

 starfish and brittle-stars large numbers of young have to encounter 

 the huge mortality of a free life in the waters, in others only a few 

 are produced, and creep about on the body of the mother (Fig. 27) 

 or develop in a brood-pouch formed on the outer surface of her body. 



FIG. 26. A Sea-urchin carrying its 

 young. (After WYVILLE THOMSON. ) 



