652 A Remarkable Life History and its Meaning. [| November, 
seen to be attached by a narrow neck to the wall (e) of the breath- 
ing chamber (h) of the nurse. In this figure, 1 represents the 
blood-channel of the chain-salpa, and the arrows show the direc- 
tion of the currents into and out of the placenta. The presence 
of a true placenta in an animal so simple in structure and so far 
removed from the mammalia is such a remarkable and interest- 
ing instance of the independent appearance of similar structures 
that a short description of it will not be out of place. It is com- 
posed of two parts: an inner chamber in direct communication 
with the blood system of the chain-salpa, and an outer chamber 
surrounding the inner but entirely shut off from it, and in free 
communication with the blood system of the foetus. 
The blood globules of the foetus are much smaller than those of 
the nurse, and may therefore be distinguished from them without 
difficulty, and after the heart of the foetus begins to beat, it is easy 
to see that there is no direct mingling of the blood of the nurse 
with that of the foetus, but simply a very close contact, exactly 
as is the case in the mammals. The large globules of the nurse 
can be seen to enter the inner chamber of the placenta, course 
around it through the intricate channels into which it is divided, 
and then leave it to return to the general circulation ; while the 
smaller globules of the foetus may be seen to make their way 
into and around the outer chamber, and then to return into the 
general circulation of the foetus. Since the reversal of the action 
of the heart of the foetus does not generally take place at the 
same time with that of the chain-salpa, the complete independ- 
ence of the two circulations is very clearly shown when either 
of them is reversed. oe 
After the embryo has acquired all the organs of the solitary 
form and has increased many hundred times in size, its attach- 
ment by the placenta to the wall of the breathing chamber of the 
nurse is broken, and the young animal falls into this chamber 
and lives there for some time, but finally escapes through the 
posterior opening into the water, and at once begins to form 
chains by budding, as already described. Figure 45 shows @ 
nearly full-grown chain-salpa, which contains a solitary embryo» 
(s). This is free within the breathing chamber, and is ready to 
be discharged. ; : 
Figures 44 and 45 are drawn to nearly the same scale, Figur 
45 being only a little more magnified than Figure 44. Figure? 
represents a solitary Salpa, which contains a chain, u, which 1$ 
ready to be discharged into the water, while Figure 45 represents 
