THE ENTBROPNEUSTA. 27 



if placed in the tissue spaces of the proboscis, are certainly 

 expelled by the pore. This evidence does not of coarse 

 demonstrate, beyond doubt, that inwardly directed currents 

 never enter the pore but only gives a presumption against 

 them. 



Spengel's statement of the absence of the pore described by 

 Kowalevsky and Agassiz, at the apex of the proboscis, is true 

 for all the species which I have examined. 



The Heart. — As mentioned by previous observers, a large 

 vesicle may be seen pulsating above the water-vessel in Tor- 

 naria; such a pulsation may be observed in the dorsal side of 

 the base of the proboscis in B. Kowalevskii at the stage of 

 two to three gill-slits. Spengel states that this contractile sac is 

 the upper of the two cavities lying above the notochord (figs. 51, 

 53, &c.) ; this he calls the " heart." In consideration of the 

 fact (which he also admits) that it contains no blood, gives off 

 no vessels, and has no muscular walls, this name seems open 

 to misconstruction. As this so-called "heart" is merely a 

 space filled with loose tissue which is part of the proboscis 

 gland I have preferred to call it the sac of the proboscis gland, 

 and to reserve the name "heart" for the sac which lies 

 between this space and the notochord (figs. 50 and 51, ht.). 

 That this is the actual heart can I think hardly be doubted. 

 It arises at about three gill-slits as a single horizontal split in 

 mesoblast between the notochord and the sac of the proboscis 

 gland. It acquires muscular walls and is always nearly full of 

 a coagulum similar to that which is found in the remaining 

 blood-vessels of the body, which can all be traced into connec- 

 tion with it. 



These peripheral vessels are (1) a longitudinal dorsal one, 

 running from the heart to the tail in the dorsal mesentery 

 from the back of the collar, and in the collar as a blood-space 

 surrounded by the perihsemal cavities (fig. 60) ; (2) a ventral 

 longitudinal vessel running from the back of the collar to the 

 tail in the ventral mesentery. These two are connected by 

 blood sinuses in the skin and in the wall of the gut. I have 

 not seen the definite circular vessel which other observers state 



