PHORONIS ARCHITECTA— BROOKS AND COWLES. 85 



resultant du dedoublement de I'orifice primitif et median, tandis que le restant <lu diverticule 

 ectoblastique deviendrait la cavite posterieure du corps." 



Such an origin for this posterior cavity would seem to be a possible one and would give an 

 easy explanation for the origin of the collar trunk and ventral mesenteries. 



We believe that the cavity of the trunk is formed in the following manner: As the tentacles 

 grow out and increase in number the posterior region of the larva about the tectum increases 

 greatly in length. In doing the latter the mesodermal lining of the collar is drawn away from 

 the somatic wall in the region back of the tentacular band, and a cavity is left containing the rec 

 turn, part of the stomach, and the proximal part of the nephridial diverticula. At the same time 

 this is taking place certain cells which seem to arise from the base of the nephridial diverticula 

 give rise to the lining of the cavity of the trunk. As to the manner of origin of these cells 

 we are still in doubt. We have not found two ccelomic sacs which Hatschek (8) seems to have 

 figured (it is possible that his figure is meant to represent a single sac cut at two places). 

 and we have hunted for them in larva 1 where the diverticula are just beginning to form and 

 also in larvaa with two, four, and six tentacles. In one specimen with two tentacles, however 

 (fig. 30), an arrangement of mesodermal cells on the dorsal side of the intestine which seems 

 to be the beginning of a sue is found; this, however, is not paired. Whether or not this sac 

 and its cavity give rise to the lining and cavity of the trunk we can not say. for we have found 

 but one specimen in which this condition exists. 



One thing is certain, the fully developed trunk cavity of the Actinotrocka has a distinct 

 mesodermal lining, consisting of a somatic and a splanchnic layer. As far as we know all Actino- 

 trochse have a ventral mesentery, which tends to support the view that the lining of the cavity 

 of the trunk has its origin in a sac which grows around the rectum and posterior part of the 

 stomach. Whether or not the fact that there is an indication of a dorsal mesentery in the pos- 

 terior region of some of the fully developed Actinotrochx, Species 15.. has any bearing on the 

 double origin of the cavity of the trunk we can not say. for we have never seen the very young 

 larva? of this form. 



The youngest larva taken from the tow had three pairs of tentacles, with beginnings of the 

 fourth pair. In this larva the tentacles had grown considerably in length, and the posterior 

 region had become somewhat elongated (tig. 31). Only one specimen of this age was obtained. 

 and it was only studied while alive. The mesentery between the collar and lobe was plainly 

 seen, and there seemed to be a thin mesentery between the region of the collar and the younger 

 trunk region. The nephridial canals were seen with difficulty, but the rounded bunches of 

 excretory cells forming the internal ends of the canals were plainly visible. 



When the larva of Phoronis architecta has five pairs of tentacles (tig. 32) the trunk region is 

 elongated considerably and constitutes about one-half the length of the larva. In fact, the larva 

 at this stage looks much like the fully developed Actinotrocha. The •"retractors"' described by 

 Ikeda (9) are now present and the body wall in the anal region shows a thickening which is to 

 become the perianal ciliated band. 'Phis larva shows clearly the presence of two mesenteries. 



In tin' larva with six pairs of tentacles (tig. 33) till of the organs of the fully developed 

 Actinotrocha arc present. The ventral pouch begins to invaginate (tig. :!:-'>) and sections usually 

 show that the blood corpuscle masses are forming. 



FULLY DEVELOPED ACTINOTROCHA. 



There are two species of Actinotrochm found in the waters of Beaufort Harbor, and they are 

 very similar, if not identical, with the two species that E. B. Wilson (24) observed in Chesapeake 

 Bay. From the latter part of May until the latter part of September both species are fairly 

 abundant in the tow. 



Wilson has designated the two species found in Chesapeake Bay as Species A. and Species 

 B., and because of the general agreement between our observations and his descriptions the 

 Beaufort Actinotrochse will be designated as Species A. and R, although we are satisfied that 

 Species A. is the larva of P. architecta. 



