102 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



vessel, and when the metamorphosis is completed ;i definite vessel is seen, which opens posteri- 

 orly into the spacious blood sinus around the loop of the alimentary canal. Anteriorly before 

 reaching the transverse septum, it divides into two branches, which run obliquely upward along 

 the sides of the alimentary canal, almost encircling the same; these finally open into the ring 

 vessel of the supraseptal cavity (fig. 60). The vessel described becomes the efferent vessel of the 

 adult (figs. 78, 77. 75, 66) and its branches become part of the recipient vessel. 



A> Ikeda has pointed out, the efferent vessel of the adult corresponds to the ventral vessel 

 which Masterman (15) and Roule (20) have found in the Actinotrocha before metamorphosis. 



In all the completely metamorphosed Aetin<itr<><Ii;< that have heen sectioned there is but one 

 ring vessel, hut the young 1 Vmrmi is, when it is 12 hours old, possesses both the recipient and 

 distributing vessels; these vessels, we believe, arise from the single ring vessel of the metamor- 

 phosing Actinotrocha by the fusion of its walls and by the subsequent separation of the two 

 parts along the line of fusion. 



Masterman, in his description of the blood system of the Actinotrocha, speaks of a "ring 

 sinus" at the anterior end of the intestine which connects the dorsal and ventral vessels. He 

 also says that there are two lateral branches of the dorsal vessel in the region of the pharynx 

 which pass downward around the oesophagus ("'post-oral ring sinus") and become continuous 

 with the ventral vessel. 



The former undoubtedly represents the sinus surrounding the loop of the alimentary canal 

 in the young Phoronis, while the latter, no doubt represents the branches of the efferent vessel 

 which become part of the recipient vessel of the adult. Masterman says that these branches 

 open into the dorsal blood vessel, but such is not the case in the completely metamorphosed 

 Actinotrocha. 



From a comparison of Masterman's description of the vascular system of the Actinotrocha 

 and Ikeda's and our own description of the same before and after metamorphosis, it is seen at once 

 that this system develops more precociously in the form that Masterman studied. This condition, 

 together with the facts that the lumen of the blood vessels are parts of spaces between the wall 

 of the gut and its mesodermal lining and that the mesodermal lining of the alimentary canal tits 

 loosely while the blood system is developing, gives additional weight to Masterman's statement 

 that the dorsal vessel opens into the so-called "subneural sinus." However, in the Actinotrochse 

 that we have examined such a connection does not exist, and, as stated above, a "subneural sinus" 

 or cavity caused by a lack of contiguity between the mesodermal wall of the preoral lobe and 

 that of the collar cavity is not present. 



THE ADULT PHORONIS ARCHITECTA. 



Phoronis architecta was discovered by Andrews (1) in June, 1SS5, at Beaufort, N. O. and 

 he described it as a new species, giving it the specific name "architecta," on account, no doubt, 

 of its building a beautiful, straight tube. He finds that the tubes are made up of a clear, firm, 

 chitin-like membrane covered with small, clear grains of sand, and he thinks that these grains 

 are selected by the animal. Specimens collected from different localities in Beaufort Harbor 

 vary considerably in regard to the character of the sand grains and quite often small fragments 

 of dark shells are found mixed in with the latter. Occasionally two tubes occur cemented 

 together; but this condition is rare, for they are usually isolated and embedded perpendicularly 

 in the sand. When the specimens are brought into the laboratory and put into aquaria with 

 sand and water, they usually ciawl out of their tubes and begin to form new ones. Long- 

 champs (13) has lately pointed out that the tube is formed by a secretion from the posterior end 

 of the animal and not from the anterior end, as Cori has said. This is the case for P. architt /</. 



Above it is stated that the tubes are straight, but where new tubes are formed in the aquaria 

 they are always twisted to a considerable extent, and they are attached firmly to the bottom 

 of the jar. In its natural habitat. Phoronis iir<-hit<<-tn does not have a firm substratum to which 

 to cement its tube, but it is seen from the above observation that when a solid surface presents 

 it-elf. til.' tube may take on the condition found in some of the other species of Phoronis which 

 are attached to rocks and shells. 



