HEMICHORDA 667 



basement membranes of the ectoderm, endoderm, and mesoderm, 

 which otherwise are everywhere in contact, having nomesenchymatous 

 connective tissue between them. A dorsal vessel above the ahmentary 



■pro. 



Fig. 467. A zooid of Rhabdopleura normani removed from its tube and seen 

 from the right-hand side. From Lang. An. anus; arm. arms; M. position 

 of mouth; por.' position of collar pore; pro. proboscis; stk. stalk; trk. trunk. 



canal widens over the notochord in the hinder part of the proboscis 

 into a space known as the heart. This is covered by a vesicle known 

 as the pericardium, whose lower wall, contracting, communicates 

 pulsations to the blood. From the heart the blood passes into a plexus 



