12 A NATURALIST ON THE "CHALLENGER." 



St. Thomas is notorious, and close to our anchorage was a portion 

 of a large iron dock which had been sunk before ever it could 

 be used. Behind the town of St. Thomas are hills rising to 

 a height of 1,400 feet at their highest point. 



I landed at one of the many wooden jetties amongst 

 numerous negroes of both sexes lolling about and chewing 

 sugar-cane, their constant occupation. The shore is covered 

 with corals bleached white by the sun, and amongst these lay 

 quantities of calcareous seaweeds (Halimeda opuntia and H. 

 tridens), branching masses composed of leaf-shaped joints of 

 hard calcareous matter articulated together. These were all 

 quite dry and bleached white, and hard and stiff, like corals. 

 Seaweeds belonging to two very different groups of algae thus 

 secrete a calcareous skeleton, Halimeda and its allies belonging 

 to the SiphonaceaB, green algse, and Lithothamnion and allied 

 genera belonging to the Corallinaceae, which are red coloured 

 algae. These lime-secreting algse are of great importance from 

 a geological point of view as supplying a large part of the 

 material of winch calcareous reefs and sand rocks are built up. 

 At St. Thomas the Siphonaceae are especially abundant, whereas 

 at other places, as at St. Vincent, Cape Verde Islands, the 

 Corallinaceae appear to supply most of the calcareous matter 

 separated from the sea water by plants. 



The rise and fall of the tide at St. Thomas is only about a foot; 

 yet along the very margin of the water 1 found plenty of animals 

 living, some of them only just awash. Sea urchins (Diadema 

 antillarum), with extremely long sharp spines, were very common. 

 The spines penetrate a bather's foot or hand with the greatest 

 facility, and breaking off leave a very unpleasant wound. In 

 gathering specimens I got wounded in the finger, though I took 

 great care ; so well are the animals protected. The animals keep 

 their long spines in constant motion, so that it is very difficult 

 to avoid being pricked if one tries to handle one. The wound 

 produced by the spines is apt to fester, but there appears to be 

 no poison on the spine. In the case, however, of another genus 

 of sea urchins which I dredged in abundance in shallow water 

 on the Philippine coast, and in which the short spines are hollow 

 and tubular at their extremities, a definite poison certainly 



