Fig. 3. 



narrowly, to form the main vessels. In a word, the heart is 

 constructed on the principle of the tunicate heart. Spengel 

 has described an essentially similar arrangement in Stereobalanus 

 [Balanoglossus) canadensis, Hill, the same thing in Balanoglossus 

 (Ptychodera) australiensis, and Benham the same, engorgement with 

 blood and all, in Dolichoglossus [Balanoglossus) otagoensis." 



I then proceed to point out that the seeming uniqueness of the 

 heart in these species is indeed seeming rather than real. That the 

 relations are in reality those of the typical enteropneust heart and 

 pericardium, the seeming difference being probably ascribable to the 

 fact that the heart of these particular individuals happened to be 

 unusually full of blood when death intervened. 



From his observations Uawydoff considers that a true homology 

 between the tunicate and entero- 

 pneust heart and pericardium should 

 be recognized, and thus that »wir 

 es hier mit neuen Thatsachen zu 

 Gunsten einer Verwandtschaft zwi- 

 schen den Enteropneusten und den 

 Tunicaten zu thun haben«. 



In my manuscript of a year 

 ago I have expressly renounced any 

 intention of trying to establish a 

 homology between the two hearts, 

 but on the contrary have pointed 

 out what I then considered as fatal 

 difficulties in the way of such ho- 

 mology. While still recognizing the 

 full force of these difficulties, far- 

 ther reflection has led me to now 

 seriously doubt whether the facts opposed to homology are after all 

 as weighty as those in favor of it. In the first place the adult structure 

 of the organ in the two animals must be admitted to be fundamentally 

 the same. That the tunicate heart has secondarily acquired a connec- 

 tion at both ends with blood vessels and has taken on the habit of 

 alternating the direction of its contractions; that the mouth of invagi- 

 nation has become more nearly closed in the tunicate than in the 

 enteropneust; and that the epithelial wall of the tunicate heart has 

 produced muscle fibers of a sort not found in the enteropneust can 

 hardly be taken to mean more than that the heart of the tunicate is 

 more specialized than that of the enteropneust. 



A few years ago the fact that the enteropneust pericardio-cardiac 



1* 



