48 ANATOMY AND rHVSIOLOGV OF SALPA AND PYROSOMA 



eventually the heart is obviously seen pulsating close behind the 

 pedicle of attachment, Plate XVI. [Plate 6] fig. 6. 



In the meanwhile the smaller subspherical mass has undergone a 

 remarkable change. It has likewise become thrust from the sinus 

 towards the respiratory cavity, so that it no longer lies in the former, 

 but is situated in the thick pedicle of the young Salpa. 



It has furthermore become hollow, and contains two perfectly dis- 

 tinct cavities or sacs ; of these the outer is concave and cup-shaped 

 and en\-elopes the inner, which is subspherical, Plate XVI. [Plate 6] 

 fig. 6 111. Now the outer sac is in free communication by a narrower 

 neck, di\-ided into two channels by a partition, with the dorsal sinus 

 of the ftetus ; and the inner sac is in equally free communication by a 

 neck similarly divided, with a short sinus arising immediately behind 

 the heart ; and as there is no communication between the two sacs, it 

 follows that the current of blood in each is perfectly distinct from and 

 independent of, that in the other. A more beautiful sight indeed can 

 hardl)- be offered to the eye of the microscopic observer than the cir- 

 culation in this organ. The blood-corpuscles of the parent may be 

 readily traced entering the inner sac on one side of the partition, 

 coursing round it, and finall)^ re-entering the parental circulation on 

 the other side of the partition ; while the foetal blood-corpuscles, of a 

 different size from those of the parent, enter the outer sac, circulate 

 round it at a different rate, and leave it to enter into the general cir- 

 culation in the dorsal sinus. 



More obvious still does the independence of the two circulations 

 become when the circulation of either mother or foetus is reversed. 



30. Whether this bod}' perform the function or not, it can hardly 

 be wrong to give it the name of a placenta. It is identical in 

 structure with a single villus contained in a single venous cell of 

 the mammalian placenta, except that in the Salpian placenta the 

 villus belongs to the parent, the cell to the foetus; the reverse ob- 

 taining in the Mammalia. 



As the young Salpa increases in size, the placenta, ceasing to grow, 

 becomes proportionately smaller, until the pedicle gradually narrowing 

 the communication with the parent ceases and the foetus becomes 

 free, Plate XVI. [Plate 6] fig. 3. The remains of the placenta are 

 traceable for some time as a small diverticulum of the dorsal sinus 

 of the young Salpa, Plate XVI. [Plate 6] fig. 3 w. 



31. The latter as it grows nowise resembles its parent. It has a 

 prismatic form, has seven muscular bands, and developes processes 

 from its posterior extremit)'. It becomes indeed perfectl)' similar to 

 the form which has been described as Salpa A. 



