pf:rophora annectens. 75 



high a grade of development as is found in anj' Tunicate, 

 I may quote Roule '85, p. 106: " Les canaux sanguins, 

 sauf le coeur, n'ont pas de parois propres, isolables du 

 tissu environnant ; les plus simples d'entre eux ne pos- 

 sedent que leur mince endothelium, et resemblent tout a 

 fait aux espaces laisses, chez les animaux superieurs, 

 entre les faisceaux du tissu conjonctif ; les plus complexes 

 ont une enveloppe musculaire, mais les elements de cette 

 enveloppe ne lui appartiennent pas en entier, et font aussi 

 partie en substratum conjonctivo-musculaire environnant. 

 Le coeur seul possede des parvis propres." This rudi- 

 mentary condition of the vessels is still more pronounced 

 in such comparatively primitive forms as Perophora, 

 where even the main vessels or merely such channels, i. e., 

 the median ventral, and the median dorsal trunks, possess 

 the thinnest, most imperfect kind of a wall of connect- 

 ive tissue fiber and endothelium. I can detect no trace 

 of muscle fibers in them. 



Considering then the tunicate blood system in the light 

 of Weber's " Schema of the Circulation," we may fairly 

 conjecture that by the retarding effect upon the blood 

 stream produced by the systemic lacunar and the branchial 

 network of small vessels, and by the absence of elasticity 

 and valves from the larger vessels, before a degree of 

 pressure was reached in the arterial system sufficiently 

 great to force the blood through the two connecting 

 systems rapidly enough to return as much blood to the 

 heart by the veins as was being forced from it into the 

 arteries, a regurgitation of blood into the heart resulted 

 which in some way produced a reversal in the direction 

 of the contractions. 



Since reaching- this conclusion concerning; the cause of 

 the reversed movements of the heart, 1 find that Roule 

 ('85) has arrived at one apparentlv somewhat similar. 



