388 LOWER FORMS OF MARINE LIFE 



of the Indo-Pacific, may be cited as an example of species in which protrusile- 

 tube-feet, like those seen on the under side of a starfish lying on its back on, 

 the beach, are developed ; these so-called feet in this particular species being 

 irregularly distributed, instead of arranged in longitudinal rows, as is more 

 generally the case. Some species entirely lack tube-feet; and in the deep-sea 

 representatives of the group such feet are present only on the under surface of the 

 body, the upper side being furnished with long conical papillae, which appear to- 

 be specially modified feet no longer used for progression. In one remarkable deep- 

 sea species known as Seotoplanes globosa these dorsal papillae are reduced to four, 

 corals and Sea- Although deep-sea corals extend from the Arctic to the Antarctic 



Anemones. Ocean, in which they may be found from depths of at least 2900 

 fathoms to less than a fathom, the reef-building types, which are those of the 

 greatest general interest, are restricted to the warmer seas, more especially the 



SCOTOPLANES GLOBOSA. 



Indo-Pacific. None of them can live at a depth exceeding 20 fathoms, or in- 

 water of which the temperature falls below 68° F. Corals are not tidal organisms, 

 and must alv/ays be covered either by the water or washed by the surf, as a very 

 short exposure to the rays of the sun is sufficient to kill the polyps. From ob- 

 servations made on the Cocos-Keeling Atoll and in the Solomon Islands it has- 

 been ascertained that corals are subject to great seasonal and individual variation 

 in their rate of increase, while there is likewise a great difference in this respect 

 between the branching and the massive groups. On the average, it appears that 

 branching corals grow about 3'7 inches in a twelvemonth, whereas the massive- 

 species increase their diameter by about one thirty-seventh of their original cir- 

 cumference in a hundred days. In other words, a coral of 37 inches in diameter 

 will measure 38 inches across in a little more than three months. 



Of the massive forms, the most abundant types are Porites, star-corals or 

 Astrcea, madrepores or Madreporaria, and, in sheltered positions, brain-corals 

 or Meandrina. These or nearl}' allied forms are to be found right through the 



