576 
MR. HUXLEY ON THE ANATOMY OF SALPA AND PYROSOMA. 
More obvious still does the independence of the two circulations become when the 
circulation of either mother or foetus is reversed. 
30. Whether this body 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 con- 
tained 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 obtaining 
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. 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. fig. 3 m. 
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 extre- 
mity. It becomes indeed perfectly similar to the form which has been described as 
Salpa A. 
It thence appears that the other half of Chamisso’s theory is also perfectly true, 
viz. the aggregate form of Salpa (Salpa B) produces the solitary form {Salpa A), and 
the circulatory system of the foetus in this case is connected with that of the parent, 
not immediately , hut hy means of a very distinct and well-developed placenta. 
Here is one very clear distinction between the two processes of reproduction. 
Are there any other differences ? To answer this question we must proceed to trace 
back both processes to their origin. 
32. It has been seen that the young Salpce B are developed by a process of gem- 
mation from the gemmiferous tube of Salpa A. Whence comes the tube itself? 
The smaller the individual of the form A examined, the shorter is the gemmiferous 
tube, and the less developed the buds upon it. In individuals just free, or about to 
be free, it is a very short cylindrical tube, arising on the right side and just in front 
of the heart, and curving downwards and backwards, Plate XVI. figs 3, 3 a. 
In still smaller attached specimens it appears as a very short, somewhat conical 
process (imperfectly divided by a partition) of the dorsal sinus, close to the heart; its 
walls are smooth, and the blood-corpuscles are easily seen passing up one side and 
down the other of the partition, Plate XVI. fig. 4. 
It is clear therefore that the gemmiferous tube is nothing more than a stolon, con- 
taining a diverticulum of the circulatory system of the parent, and the whole process 
of reproduction as it is manifested in Salpa A is one of gemmation. Salpa B is a bud 
of Salpa A. 
33. Following the same course of investigation with regard to the young Salpa A 
(which it has been seen is produced from Salpa B), it is found, that in Salpce B, 
which are either still adherent to the gemmiferous tube or just set free, there is no 
protuberance of the inner tunic into the respiratory cavity ; but where this afterwards 
exists, a pedicle of greater or less length is attached, and running backwards, carries 
