62 Papers from the Marine Biological Laboratory at Tortugas. 



in the 2o-tentacle stage as outgrowths of the body-wall just beneath 

 the tentacles and with their axis perpendicular to the oral plane. The 

 second pair of fronds appear in the 28-tentacle stage or later. 



SUMMARY. 



(1) Convergence in structure and habitat is the cause of commen- 

 salism between an Alpheid and a Pontoniid living in the loggerhead 

 sponge. 



(2) Abbreviation of its pelagic life accounts for the numerical super- 

 sedence of the Alpheid. 



(3) The weed-like outgrowths or fronds of a reef anemone, Cradactis, 

 probably hide it from its prey. 



(4) Cradactis is kept just within the mouths of cavities in reef rocks 

 by the combined action of negative heliotropism of its base and positive 

 heliotropism of the fronds. The fronds are entirely contracted in the 

 absence of light. 



(5) The fronds possess the sense of taste but do not carry food to 

 the mouth. 



(6) Cradactis moves from place to place by walking on its tentacles, 

 a phenomenon sometimes seen in Hydra. 



DESCRIPTION OF PLATES. 



("Figures 4 and 5 were redrawn by Mr. Kline, fig. 6, from life, by ? K. Morita, 

 otherwise the drawings and photographs are the author's.) 



PLATE i. 



1. Synalpheus brooksi Coutiere. 



2. Chela of same species to show different coloration. 



3. Typion tortuga Rathbun commensal with the above. 



4. Cradactis variabilis Hargitt. 



5. The same, showing another variety in color and shape of fronds. 



6. Cradactis variabilis Hargitt, X 2. 



PLATE 2. 



7. A portion of an old coral head showing the fronds (f) of Cradactis protruding 



from the cavities. 



8-1 o. Cradactis variabilis, showing varieties in shape of fronds, 

 u, 12. Cradactis variabilis, walking on its tentacles, with detached base toward 



the observer. 



