HEAET-BODY, ETC., OF CERTAIN POLYOH^TA. 293 



appeared as a cluster of cells with large nuclei in the dorsal 

 vessel. From the first it shows signs of pigmentation. Even in 

 the living state a cavity can be recognised in it, whilst sections 

 show that part at least of this cavity opens directly into the 

 coelom on the ventral side of the heart just anterior to its origin. 

 In other words, the heart-body is an in-pushing of the heart- 

 wall (fig. 36). It shows no connection whatsoever with the 

 hypoblast. Later (fig. 35) the open connection with the coelom 

 appears to be narrowed, and finally obliterated. 



Sections through the organ at the stage when this is going 

 on are difficult to interpret except in the light of the earlier 

 stage. Nevertheless the heart wall may be traced on the 

 ventral side to a point where it is invaginated, and becomes 

 continuous with the wall of the heart-body (x.). It is not clear 

 whether the lumen which primitively exists at the posterior 

 end of the heart-body, and which at first is open to — in fact, is 

 a part of — the coelom, is continuous with that in the anterior 

 part of the organ. It is not unlikely that, an invagination 

 of the heart-wall having taken place, proliferation of cells 

 from the anterior end of the invagination gives rise to the 

 anterior part of the organ, and that in this a secondary cavity 

 is formed. 



However that may be, the main fact is evident that the 

 heart-body in Polymnia is a purely mesoblastic structure. 



The Heart-body in the Amphictenidae. 



In Pectinaria, Claparede (9, A, p. 379) states, " Le gros 

 vaisseau intestinal est accompagne dans toute sa longeur par un 

 cordon jaune, irregulier, adherant a sa parol, et forme de cel- 

 lules larges de 11 micr., et pourvues d'un nucleus clair. Get 

 organe est sans doute comparable aux boyaux celluleux qui 

 adherent au vaisseau dorsal chez tant d'Annelides. J'insiste 

 sur ce fait, parceque ce vaisseau dorsal de I'intestin joue chez les 

 Pectinaires le role d'un vaisseau dorsal veritable.^' 



Wiren (33) , in his figure of the blood-system of Pectinaria, 

 shows that this great intestinal vessel gives off the gill vessels. 



