Adaptations in Structure and Habits of Certain Marine Animals. 61 



the disk running towards the mouth. A secretion sticks the particles 

 together. These currents are useful on both fronds and tentacles in the 

 rejection of particles, and on the tentacles in the placing of food in the 

 mouth, the food being carried to the tip of the tentacle before it is placed 

 in the mouth. 



When disturbed by light falling on the base, it sometimes moves 

 with snail-like motion (like Metridium) a short distance, but the tentacles 

 catch hold of the substratum on all sides. The tentacles and column 

 sometimes perform writhing movements. More often the animal bends 

 over to one side and catches hold of the substratum with the tentacles, 

 with or without previously elongating the column, the fronds contract- 

 ing slowly all the while. It then loosens the base, walks on its tentacles 

 to a new place (plate 2, figs, n, 12), bends over and attaches the base, 

 and lets go its hold with the tentacles. This method of locomotion is 

 much more rapid than that of Metridium, but could not be used if the 

 Cradactis did not live in holes, as it might otherwise be washed away 

 by the currents that constantly sweep over the reefs. 



The resemblance of the fronds to sea-weed leads one to suppose that 

 they act as lures or in hiding the Cradactis from its prey (anemones 

 being unpalatable are usually not in need of protection). The fact that 

 the fronds are heliotropic and contracted completely at night is in har- 

 mony with this view. I did not 

 cut them off to see whether the 

 anemone would live and repro- 

 duce as well without them. The 

 cavities containing the Cradactis 

 are inhabited by other animals, 

 especially a small black crab, 



i 1 , ., FIG. i. Cradactis variabilis. a, Planula just escaped 



and One might Suppose that the from ccelentric cavity of mother, oral (pigmented) 



frnnrk nrntppfpH flip fpnta/-W nf side u PP ermos t- b, The same, second day, seen 



IS protected tlie tentacles OI f rom oral side; pigment arranged radially, c, The 



the anemone from tVlP IPOX nf fhp same, third day. d. The same, fourth day; ten- 



tacles elongating and septa becoming distinct. The 



Crabs that Crawled Over it The mouth should be elongated in the plane of sym- 



metry. 



crabs are active at night in the 



least light in which they can be seen (their black color making them 

 hard to see in the holes in the rock) . In case they are normally active 

 at night the fronds would serve as a protection from the crabs only half 

 of the time. The anemones sometimes grasp the crabs and hold them 

 until they wrench themselves loose, which they invariably do in a short 

 time. Perhaps the anemone gets part of its food as particles dropped 

 from the crabs' mouths. 



Cradactis develops to the planula stage in the ccelenteron of the 

 mother. On being released, the planula swims around for a few hours 

 (text fig. i, a) and attaches itself (6) by the smaller end. It gradually 

 develops a mouth and tentacles (b-d). When first liberated, the planula 

 has 8 mesenteries, and 8 tentacles develop soon after. Individuals were 

 seen with 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28 and more tentacles. 

 From this one might" conclude that the tentacles (and mesenteries) 

 appear in pairs, but they ware often observed to appear in sets of four, 

 symmetrical in relation to the oral plane. The first pair of fronds appear 



