GROWTH OF YOUNG STAR-FISHES. 



125 



A MORE ADVANCED EMBRYO, 

 SWIMMING BY BOWS OF CILIA 

 ALONG THE PROTUBERANCES. 



treacherous small currents that lead into an oyster's, clam's, or mussel's 

 mouth. This helps to even up the account which the adult star-fishes 

 are making by their daily onslaughts upon these same mollusks. 



The jaunty, free career of the brachiolaria, however, is soon over. 

 Changes begun before they were understood now begin to alarm him. 

 He is losing his shape and assuming a strange- 

 ly symmetrical, five-armed form, covered with 

 soft spines and tentacles. Before he knows 

 it, and without the loss of a single portion, the 

 brachiolaria, by absorption, has lost himself in 

 the body of a true young star-fish, and finds 

 himself slowly acquiring the stiff armor and 

 dignified mien which marks his approach to 

 an adult condition. He ceases his gay wan- 

 derings and sinks to the bottom, or crawls 

 upon the frond of some floating sea-weed. 

 This occurs when he is about three weeks 

 old. But now that he is no longer an em- 

 bryo, but a real baby star-fish, his growth is 

 very slow. Mr. Agassiz says that by arrang- 

 ing the star-fishes, big and little, found upon 



our rocks, into series according to size, we may roughly estimate the 

 number of years required by them to attain their full development, 



which he thus calculates to be about fourteen. 

 During the earlier years the growth is more 

 rapid, of course, than later. One young speci- 

 men, kept in an aquarium at the Cambridge 

 museum, doubled its diameter in five months. 

 That they begin to spawn when six or seven 

 years old, or hardly half-grown, is ascertained ; 

 but as to how long they may live after that, 

 provided the dangers of the sea are escaped, 

 we have no information that I am aware of. 

 The size to which they attain varies in 

 different species. The rare British Uraster 

 glacialis, Ag., has been seen thirty-three inches in diameter, and some even 

 larger than this have been reported from near Eastport, Maine, where 

 echinoderms abound in greater number, perhaps, than anywhere else on 

 our coast.. South of Cape Cod, however, it is rare to see one measuring 

 more than ten inches across, and the great majority do not exceed six. 



A YOUNG STAR-FISH. 



