370 TUNICATA. 



vaginated part of the wall of the vesicle becomes the wall of the 

 heart while the non-invaginated part changes into the pericardial 

 epithelium. 



The lumen of the heart is thus, according to the above description, 

 a cavity which has arisen through the invagination of the outer 

 surface of the pericardial vesicle. This cavity communicates, by 

 means of the long dorsal aperture of invagination, with the lacunar 

 blood-spaces of the surrounding mesenchyme. This communication 

 is partly closed later by a lamella arising from the epicardium ; it 

 is, however, retained at the anterior and the posterior ends as the 

 anterior and posterior apertures of the cardiac tube. 



That part of the procardial rudiment which remains after the 

 abstriction of the pericardial vesicle is from that time known as the 

 epicardium. It consists, as before, of a posterior unpaired diverticu- 

 lum (sac epicardique) which forks anteriorly into two paired epicardial 

 tubes (tabes epi car cliques) ; these, in their turn, entering the pharynx 

 to the right and left of the median line (Fig. 171 A). The point at 

 which they enter lies between the posterior end of the hypobranchial 

 furrow and the point of entrance of the oesophagus. The posterior 

 caecum of the epicardium now grows out backward considerably, and 

 thus reaches the dorsal side of the heart-rudiment (Fig. 173 (7, ep 

 and h, p. 375) with which it comes into such close contact that its 

 ventral wall is drawn in to close the dorsal aperture of the heart. 



The epicardium is a structure of great significance in those forms 

 which reproduce asexually, being intimately connected with the pro- 

 duction of the buds. By extending farther and farther backward it 

 reaches the stolon (Fig. 173 C, st) in which it forms a transverse 

 partition. In this process it becomes so much compressed dorso- 

 ventrally that its two layers come into close contact and (in Clavelina) 

 fuse completely. The epicardial transverse partition separates, in the 

 stolon, two blood-spaces in which the blood flows in opposite direc- 

 tions. As this partition- wall does not reach quite to the blind end 

 of the stolon (Fig. 229, x, p. 456,) the two blood-spaces pass into 

 each other at this point. We shall have to return later (p. 450) to 

 the significance of the epicardium in connection with the development 

 of buds. 



The wall of the heart consists of pavement-epithelium directly 

 connected with the pericardium, and bears to the latter the same 

 relation as exists between the visceral and the parietal layers of a 

 vertebrate pericardium. In later stages, the cells of the wall of 

 the heart secrete, on the surface turned to the lumen of the heart, 



