600 



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 evagina- 

 tions of its splanchnic mesodermal covering; a loose sac of mesodermal 

 tissue (at the time of metamorphosis) enclosing the larval collar-cavity 

 and lining the somatic wall of the same and finally prolongations of 

 the sac into the tentacles as the adult tentacular blood vessels (Fig. 1 

 e.c.Z, t.b.v, Fig. 2 e.c.l). 



There are several important points concerning 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 mesodermal lining of the trunk-cavity, encloses a part of 

 the space between the lining and the wall of the alimentary canal, i. e. 

 the segmentation cavity; second, that this vessel ceases to exist as 

 such posteriorly but opens into the space between the lining and the 

 Avail of the alimentary canal; third, that the wall of the stomach in the 

 collar segment is practically free from a mesodermal lining (Fig. 2) 

 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 segmen- 

 tation cavity; and fifth, that during metamorphosis, the act of drawing 

 the stomach and intestine into cavity of the ventral pouch causes a 

 pressure to be exerted on the larval collar-cavity. 



When the critical stage in the metamorphosis is being passed 

 through, the blood corpuscle-masses break up and 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 the trunk-segment becomes 

 separated from the wall of the alimentary canal and thus allows the 

 blood corpuscles to move about between these two layers throughout 

 its extent. 



The dorsal blood vessel (»Mediangefäß« [Cori^], »afferent vessel«, 

 [Benham^]) and the ring-vessel with its tentacular vessel are complet- 

 ely formed structures at this stage. The dorsal vessel is still freely 

 open posteriorly into the space or sinus between the stomach wall and 



5 Zeitachr. f. -wise. Zool. Vol. 51. 1891. 



6 Quart. Journ. Micr. Sc. Vol. 30. 1889. 



