IX 



ENTEROPXEUSTA— BLOOD VASCULAR SYSTEM 



583 



simpler, and its sinus, wliich anteriorly is so complicated, becomes on each side a 

 simple vessel in the limiting membrane between the pioboscidal intestine and the 

 visceral coelomic endothelium. 



These two vessels are the efferent proboscidal vessels. 



Blood reaches the central sinus of the proboscis in the following ways : (1) 

 from the dorsal vessel of the trunk and collar through the basal blood sinus, 

 (2) out of the integumental vascular network of the proboscis through vessels or a 

 vascular plexus, which ascends in the limiting membrane of the ventral septum, 

 and lastly (3) out of this vascular network through a vessel, which descends along 

 the free edge of the " heart vesicle." 



The "heart vesicle" propels the blood by means of its ventral wall, which lies 

 upon the central sinus. Its function is considered to be that of driving the blood 



Pig. 464.— Diagrammatic transverse section tlirougli tlie proboscis of an Enteropneustan. 

 1, Dorsal proboscidal .septum ; 2, proboscidal epithelium ; 3, blood laeunffi of the integuuient ; 4, 

 circular musculature ; 5, longitudinal musculature ; 6, ventral proboscidal septum ; V, proboscidal 

 diverticulum of the buccal cavity ; S, proboscidal cffilom ; 0, central blood sinus of the proboscis ; 

 10, proboscidal glomerulus ; 11, " heart vesicle." 



through the narrow passages of the glomerulus, and through the efferent proboscidal 

 vessels finally into the ventral vessel of the trunk. 



If the proboscis pore takes in water for swelling the proboscis, it appears 

 pretty certain that the glomerulus (formerly called the proboscidal gill) must, 

 irrespective of other unknown functions, also serve for respiration. 



5. The efferent proboscidal, and collar, vessels.— From their origin out of the 

 two posterior tips of the lateral portions of the glomerulus, these vessels turn 

 ventrally, and, running very close to one another, traverse the chondroid tissue of 

 the neck. In the anterior and upper area of the collar region, they enter the two 

 vascular folds, in whose limiting membranes they run, breaking up into more or 

 less rich plexuses. Their courses then, naturally, correspond with those of the 

 vascular folds, which take their name from the vessels within them. In ScMzo- 

 cardiam, Balanorjlossus, and Glandiceps, where the two vascular folds descend 

 slantingly to the ventral jnedian hne of the posterior end of the collar, tliey neces- 



