PHORONIS ARCHITECTA— BROOKS AND COWLES. 97 



At the time of metamorphosis in the Actinotroohae Species A. and Species B., there is no 

 sign of a ventral blood vessel along- the stomach, such as Masterman (15) and Roule (20) 

 describe. 



We have been unable to find the "ring sinus" which, according to Masterman, connects the 

 dorsal vessel with the ventral vessel at the end of the stomach, nor have we seen the "postoral 

 ring sinus" connecting the dorsal vessel with the ventral vessel. 



Masterman's "postoral ring sinus." •■ventral blood vessel," and •"ring sinus" (situated at 

 the junction between the stomach and intestine) will l>e discussed in the section on the metamor- 

 phosis. 



There is undoubtedly a space between the wall of the perianal ring and its mesodermal lining 

 (tig. 49) in preserved specimens which seems to be what Masterman calls the " haemal ring." 

 but it does not become any organ of the adult. 



As stated above, we believe with Wilson, Caldwell, and Ikeda that the cavity of the collar 

 and its somatic mesodermal lining become the ring vessel of the adult. 



We shall continue the discussion of the further development of the dorsal blood vessel into 

 the efferent and afferent vessels of the adult in the section on the metamorphosis. 



Masterman speaks of haemal sinuses passing down the tentacles, but says that they are not 

 very decided. Ikeda has. however, investigated these structures carefully, and we thoroughly 

 agree with his view that the cavity of the collar, together with its somatic lining, extends into 

 the tentacles, and that these prolongations become the tentacular vessels of the adult. This 

 condition is shown very plainly in a dorso-lateral section of the Actinotrocha Species A. (tig. 50). 



Ill, „h! corpuscles and their origin. — E. R. Wilson (2-4) has touched upon the origin of the 

 blood corpuscles, and according to him they develop in solid masses adhering to the stomach 

 walls near the base of the tentacles. Caldwell (:!) finds that the corpuscle masses "arise from 

 the mesoblast cells in front of the septum," but he has nothing further to say about their 

 position or origin. Ikeda (9) describes the blood corpuscles as arising from "gigantic meso- 

 blast cells in the body cavity of the larvae with one or two pairs of tentacles." Since the 

 publication of this paper, Ikeda has rejected this view, although he has published nothing on the 

 subject. Menon (17) thinks that the blood corpuscles arise from the splanchnopleure covering 

 the stomach and its diverticulum. According to Cori (1), the blood corpuscles in the adult are 

 formed from the endothelium of the blood vessels. 



In the Actinotrocha Species A. (probably that of Phoronis architecta) the blood corpuscles 

 usually make their appearance during the 14-tentacle stage, as in "Type A" described by Ikeda, 

 although we have found larva' of this stage in which definite blood corpuscles were not present. 



Actinotrocha Species A. with It! tentacles invariably has blood corpuscles, and they are 

 present in the so-called collar cavity as two masses more or less closely applied to the ventro- 

 lateral walls of the stomach (figs. 51 ,/. /<). In some cases, however, they are separated from the 

 wall by a considerable space. 



The transverse section of a larva with 12 tentacles in a plane just posterior to the base of 

 the tentacles, but anterior to the mesentery, always shows two masses of cells bilaterally placed 

 and closely applied to the mesoderm lining the ventro-lateral somatic wall (tig. 53). Occasionally 

 cells are found in these masses, situated very close to the mesodermal lining, which are 

 decidedly spindle-shaped in form and whose nuclei resemble those of the cells of the mesodermal 

 lining, both in shape, size, and internal structure. These cells are not very rich in cytoplasm. 

 Most of the cells, however, are almost three times the size of the cells lining the somatic wall, 

 the cytoplasmic part of the cell having increased in size to a greater extent than the nucleus. 

 Most of the nuclei have large deeply staining nucleoli (tig. 51). 



In some specimens parts of these masses of cells are apparently in the act of wandering 

 across the body cavity to the position the blood-corpuscle masses occupy in the fully formed 

 Actinotrocha. 



Some 15 or 20 larva' with 12 or 11 tentacles have been sectioned, and with one exception we 

 have found that when the mesodermal masses are present on the ventral body wall there are no 

 blood-coi'puscle masses present in the larva, and that when the blood-corpuscle masses are 



