PHORONIS ARCHITECT A— BROOKS AND COWLES. 101 



section through the anterior end of a young Phoronis and which shows a section through the 

 young nephridium of the adult, does not prove that he is dealing with the same structure. 



Vascular system. — It will be remembered that the vascular system of the fully developed 

 Actmotrocha Species A. consisted of a dorsal blood vessel (tigs. 51/'. g, //) running along the 

 median line of the stomach from the dorsal insertion of the mesentery, between the collar and 

 trunk, to the posterior end of the stomach, its lumen being a part of the segmentation cavity; a 

 bunch of blood caeca formed at the posterior end of the stomach as evaginations of its splanchnic 

 mesodermal covering and a loose sac of mesodermal tissue arising on the somatic wall of the collar 

 segment and inclosing the larval collar cavity (tigs. 50, 51 f, </. //). (See below for discussion of 

 the " post-oral ring sinus," ventral vessel and the "ring sinus" at the junction of the stomach 

 and intestine.) 



There are several important points in the vascular system of the Actinotrocha which must 

 be taken into account in order to understand its metamorphosis into the vascular system of the 

 young Phoronis. First, that the dorsal blood vessel, which is formed from the splanchnic meso- 

 dermal lining of the trunk cavity, incloses a part of the space between the lining and the wall of 

 the alimentary canal — i. e., the segmentation cavity — second, that this vessel dwindles away pos- 

 teriorly and opens into the space between the lining and the wall of the alimentary canal; third, 

 that the wall of the stomach in the collar segment is practically free from mesodermal lining 

 (tigs. 51 ij. /i). and that the larval collar cavity, with its somatic mesodermal lining, is a blood 

 sinus; fourth, that the larval collar cavity is a part of the segmentation cavity; and, fifth, that 

 during metamorphosis the act of drawing the stomach and intestine into the cavity of the ventral 

 pouch causes pressure to be exerted on the larval collar cavity. 



When the critical stage is being passed through, the blood-corpuscle masses break up and 

 they are driven by the pressure on the collar cavity to the points of least resistance. As a rule 

 some of the blood corpuscles are squeezed up into the dorsal region of the collar cavity where 

 the dorsal blood vessel ends, and invariably some of the blood corpuscles pass from the larval 

 collar cavity into the cavity between the wall of the alimentary canal and its mesodermal covering. 

 In fact, as soon as the critical stage occurs, the splanchnic mesodermal lining in all regions 

 becomes separated from the wall of the alimentary canal and thus allows the blood corpuscles t > 

 move about between these two layers throughout the extent of the alimentary canal. 



The dorsal blood vessel (" mediangefasz" (Cori), "afferent vessel*' (Benham), and the ring 

 vessel with its tentacular vessel are completely formed structures at this stage. The dorsal 

 vessel is still freely open posteriorly into the space or sinus between the stomach wall and its 

 mesodermal covering and blood corpuscles are carried back and forth from it to the sinus by the 

 contraction and expansion of the former. Anteriorly the dorsal vessel can plainly be seen 

 opening into the ring vessel (larval collar cavity.) 



The origin of the connection between the dorsal vessel and the ring vessel and the manner 

 in which the blood corpuscles find their way into the former are questions which have not been 

 very satisfactorily elucidated. Actiiiotrni-lm Species A., does not present any great difficulties 

 in the way of understanding how these processes take place. The dorsal blood vessel opens 

 posteriorly into the sac-like sinus around the loop of the alimentary canal, and it seems probable 

 from an examination of sections of the critical stage that it is also open anteriorly. Assuming 

 that such is the condition, it will open into the space between the mesodermal lining and the wall 

 of the gut. This space, however, is in free communication with the larval collar cavity (adult 

 ring vessel) which contains the blood corpuscles. Under these conditions the blood corpuscles 

 can pass into the dorsal blood vessel from either end. 



Masterman (15) and Roule (20) both describe a vessel on the ventral stomach wall of the 

 Actmotrocha. We have not found this vessel in the Actinotrocha, nor do we find it in sections of 

 the critical stage. 



At this time there is but one ring vessel in the supraseptal cavity, but we consider that it 

 represents both the receiving and distributing vessels of the adult Phoronis. 



Shortly after the critical point in the metamorphosis, the mesodermal lining on the left side 

 of the oral limb of the U-shaped alimentary canal begins to show indications of becoming a blood 



