MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 99 



liable contribution to the subject. The behavior during metamorphosis of Actinotrocha Species 

 A. and Species B. from Beaufort Harbor seems to be quite the same as that <>l' the two Actino- 

 trocha which Wilson has observed, and there is little doUbt but that thej are of the same species. 



A.s Wilson has stated, the metamorphosis of Actlnotrocha Species A. (fig. 34) takes place 

 much more quicklj than that of Species B. (fig. 35). In fact, we have never obtained a completely 

 metamorphosed specimen of the latter, although many times we have found specimens of this 

 species with the ventral pouch well evaginated (fig. ">'>). We have tried to make the conditions 

 favorable for the completion of the metamorphosis by covei ing the bottom of the aquarium with 

 a layer of sand rich in diatoms and also by changing the water frequently. Under these 

 conditions the larva' (Species B.) would invariably sink to the bottom and move around on the 

 sand apparently in search of a favorable place to finish the metamorphosis. The latter never 

 occurred, however, although sometimes the Iarvas would attach the end of the ventral pouch to 

 the bottom of the dish. In this way the creature would often remain for days and although the 

 preoral lobe and larval tentacles would degenerate the anal end of tin 1 larva would never become 

 turned upward so as to lie in close proximity to the mouth. 



A- we have said before, we are inclined to think that the Actinotrocha Species B. belongs to 

 an adult which lives under different conditions from that of Phoronis architecta, and we should 

 not be surprised if it were found to be the Actlnotrocha of a deep-water form. Although 

 ( ', rianthus occurs in Beaufort Harbor, we have never found Phoronis australis associated with it. 



Actinotrocha Species A., a- a rule, metamorphoses in about twenty minutes (figs. 56, 56a, 

 ."it',/.), and usually just before this takes place it sinks to the bottom of the dish, but occasionally 

 metamorphosis occurs on the vertical side of the dish near the surface of the water, the young 

 Phoronis remaining fixed where the metamorphosis takes place. 



/',; oral Inh, ,n,, 1 1, ntacles. Usually the larval or distal part of the tentacles (Species A.) and 

 the preoral lobe are swallowed during metamorphosis. The proximal parts of the tentacles 

 become directed upward and constitute the tentacles of the adult. They always number IS in 

 the very young Phoronis (Species A.) and there is an indication of the horseshoe arrangement 

 which is found in the adult (fig. 565). 



The preoral lobe does not give rise to the epistome of the adult, for as Menon (17) has 

 correctly observed, this structure is not present in the very young Phoronis. However, the 

 epistome, which is of ectodermal origin, soon makes its appearance, and when the creature has 30 

 tentacles it is a very conspicuous organ (fig. 57). 



Ganglion. The ganglion on the dorsal surface of the hood is lost when the preoral lobe is 

 swallowed, and hence does not give rise to the so-called brain ganglion of the adult. 



Ectodermal wall of. collar. Although the preoral lobe degenerates, the w r all of the collar 

 does not, but becomes drawn inside the body of the young Phoronis and forms the wall of the 

 oral end of the gut. 



Perianal ciliated ring and ectodermal wall of the trunk. When the critical point in the 

 metamorphosis is reached that is. when the posterior end becomes drawn up to the region of the 

 mouth the perianal ciliated ring is usually seen as a protuberance in that region (Wilson's tig. 

 1l'). but shortly after this it becomes drawn in, and, together with someof the ectoderm, becomes 

 the lining of that part of the rectum which is near the anal opening. The drawing in takes place 

 to such an extent that most of the ectodermal wall of the trunk of the Actinotrocha becomes 

 incorporated in the wall of the rectum, as Caldwell has observed. 



This process, together with the drawing in of the ectodermal wall of the collar to form the 

 wall of the oral end of the gut, seems to cause a change in the position of the nephridial canals. 

 (See section on nephridia.) 



( 'avity "ft!,. /,,;<,r<il lnh. .Since the preoral lobe is lost d urine- the metamorphosis, its cavity 

 does not take part in the structure of the adult. 



Mesentery between //" lob< and cottar. This mesentery does not persist. 



Larval collar cavity. As has been stated by other investigators, and as we have observed, the 

 larval collar cavity with its mesodermal wall becomes the ring vessel of the adult. This organ 

 will be discus>ed further in the section on the vascular system. 



