202 EDMUND B. WILSON. 
The Or1GIn and S1GNIFICANCE of the METAMORPHOSIS of 
ActinotrocHa. By Epmunp B. Witson, Fellow in 
Biology, Johns Hopkins University. With Plates XIV, 
4 e 
Axouvt thirty-five years ago Johannes Muller described a 
very curious free-swimming pelagic animal which he cap- 
tured at the surface of the sea at Heligoland, in the North 
Sea, giving to it the name Actenotrocha, in allusion to its 
beautiful circlet of ciliated swimming arms. The striking 
appearance, and very peculiar structure of this creature sub- 
sequently attracted the attention of many observers. Wagener 
described its anatomy, supposing it to be an adult animal 
Krohn subsequently discovered, however, that Actinotrocha 
passes by a sudden metamorphosis into a Gephyrean worm ; 
and some years later Schneider made out the true nature of 
this very remarkable process. The life-history was at length 
completed by Kowalevsky, who, by raising the eggs, proved 
that the adult worm is the singular Gephyrean Phoronis, 
which had long been of especial interest as forming a sup- 
posed transition from the Polyzoa to the Annelides. The 
matter was then carefully revised by Metschnikoff, who 
studied another species found in the Mediterranean Sea, and 
published in 1871 a valuable paper setting forth the results 
of his observations. 
No attempts were made in these papers to offer any ex- 
planation of the origin and significance of the singular 
change undergone by the larva in its passage to the adult 
form. I was thus led, during the summer of 1879, while 
enjoying the facilities afforded at the Chesapeake Zoological 
Laboratory, to undertake a renewed study of the subject 
with two species of Actinotrocha commonly occurring in 
Chesapeake Bay. As a result of this study, an hypothesis 
has suggested itself that seems to me to afford a reasonable 
explanation of the steps by which the adult structure and 
strange metamorphosis of Phoronis may have been acquired. 
Moreover, a study of this particular case places in a very 
clear light some of the phenomena of metamorphosis in 
general, and is conducive to an understanding of the causes 
which have led to certain remarkable methods of develop- 
ment in a number of animal groups. In many cases of 
metamorphosis the phenomena of growth are so complex 
that it is extremely difficult to form any definite conception 
of the exact way in which they have been brought about. 
