98 MEMOIRS NATIONAL ACADEMY OF SCIENCES. VOL. X, NO. 4. 



present there are no mesodermal masses. In this exception small blood-corpuscle masses were 

 found applied to the stomach wall, and masses of cells bilaterally placed were found on the 

 ventral somatic wall, but these cells had already taken <>n the character of blood corpuscles. 



Ikeda (9) has described a "mesoblastic cell mass" which he evidently considers as giving 

 rise to the adult body cavity, and its position is very similar to that of the mesoblastic masses 

 described above. They are both products of the mesoblastic lining of the ventral somatic wall 

 and are situated between the plane of the bases of the tentacles and the plane of the somatic 

 insertion of the mesentery between the collar and trunk. Although Ikeda does not touch upon 

 the very early origin of the adult body cavity, yet it seems probable that he considers it as arising 

 from a single mass of cells. The mesoblastic masses described above are paired and bilaterally 

 placed, and they are present only in the young larva of L2 or 14 tentacles. Furthermore, in 

 the larva with 12 or 14 tentacles there is no sign of the beginning of the adult body cavity. 

 Although these mesodermal masses which, according to our observations, give rise to the blood 

 corpuscles have a similar position to the fundament of the young adult body cavity, yet we are 

 convinced that they do not give rise to it. 



In Species A. there is no intimate relation between the masses of blood corpuscles and the 

 nephridia. such as has been described by Masterman (15) for the'species from St. Andrews Bay, 

 and by Longchamps ( L2) for Actinotrocha branchiata. In the larva of 16 tentacles the blood-cor- 

 puscle masses are. however, closely applied to the stomach wall in the region of the digestive 

 area. There is no mesodermal epithelium covering that part of the surface of the stomach which 

 lies within the collar cavity, and the blood corpuscles seem to be so intimately related to the 

 digestive areas that we are inclined to believe that they receive nourishment from them. 



While the blood corpuscles vary in size and undoubtedly multiply by karyokinetic division, 

 yet we have never found the "large and somewhat coarsely granular" and the •"smaller finely 

 granular" corpuscles that Ikeda (!>) speaks of. nor in this species have we found any "gigantic 

 mesoderm cells" in the region of the blood-corpuscle masses. Very large cells in close relation 

 to the blood corpuscle masses are found in some specimens of Actinotrocha Species B. (tig. 44/'). 

 These cells resemble the cells described in the old gastrula of Species A. as arising from the wall 

 of the archenteron, only they are not as coarsely granular as the latter. While in Actinotrocha 

 Species B. the cells are found in most cases closely associated with the blood corpuscles, we have 

 never seen them in the process of division and do not believe that they give rise to blood 

 corpuscles. Their occurrence is quite variable, but as far as has been observed they are not 

 present in the Actinotrochst which are ready to metamorphose. They are not phagocytes, nor are 

 they pigment cells, and the only name which we feel justified in giving them is large free 

 mesoderm cells. Frequently they are also found in the posterior end of the trunk cavity (fig. 41/). 



Roule (-20) holds that the nephridia end internally at the level of the (esophagus, and he 

 shows this in a figure. We have made cross sections through this region and have found masses 

 of cells in much the same place as Roule has shown. These cells seem to be blood corpuscles, 

 but very few specimens have been examined, and only one of these showed these masses of cells. 



Rudiment <>ff/i< ■■mini! collar cavity" (Ikeda).— Ikeda has observed a mesodermal cell mass 

 on the ventral somatic wall just under the second tentacle in rather young specimens of all the 

 Japanese Actinotrocha. He has traced the development of this mass of cells and finds that a 

 cavity arises in it which, before metamorphosis, becomes quite spacious and extends into the 

 tentacles. We are able to confirm Ikeda's view that this cavity is the rudiment of the "adult collar 

 cavity," or "supraseptal cavity" of the adult, as it is usually called (tigs. 50, 48, 51//. 45/'. 46). 



METAMORPHOSIS. 



Several investigators have carefully described the external characteristics of the metamor- 

 phosis of the Actinotrocha, so it is unnecessary to enter into a detailed description. 



Wilson (24) studied the metamorphosis of Act) notrnrhte Species A. and Species B., which are 

 found in Chesapeake Bay. but he did not cut sections of his material. Ikeda (9), however, has 

 investigated the internal changes which take place during metamorphosis and has added a val- 



