646 A Remarkable Life History and its Meaning. [November, 
the pulsations of the heart may be seen without difficulty, and 
with a microscope the circulation of the blood may be traced to 
all parts of the body. After beating regularly for some time 
the heart suddenly stops, and for an instant the blood of the 
whole body comes to rest ; this stoppage does not last for more 
than a second, and the pulsation and circulation then recommence 
as vigorously as before, but in the opposite direction, so that the 
blood-channels which before served as arteries and carried blood 
from the heart now perform the function of veins. 
More careful examination will show what is possibly the rea- 
son of these changes. The blood does not circulate in true ves- 
sels with distinct walls, but in the spaces between the various 
organs of the body ; thus it often happens that a space or si- 
nus may have a large passage leading to it on one side and a 
very small one on the other, and the blood which enters the 
chamber through the large passage, being unable to escape with 
equal rapidity through the small one, soon accumulates and 
forms a dam or obstruction. As soon as the current is reversed, 
this obstruction is, of course, driven away from the small opening 
and gradually discharged again through the large one. Another 
peculiarity which Salpa shares with the other tunicates is the 
presence of an outer shell or test containing “ cellulose.” Cel- 
lulose is the substance which forms most of the tissues of plants 5 
and although it is almost universally present in vegetables, it 18 
found in only a very few animals, and is often stated to be one 
of the features which distinguish the vegetable from the animal 
kingdom. Salpa, however, together with a few other animals, 18 
partially composed of true cellulose. In Figures 43, 44, and 45 
the cellulose test, a, is shown as a thick transparent shell or 
outer tube, covering the remaining organs of the body. . 
By far the most interesting peculiarity of our animals is that 
the two forms, which are always found in the same locality, are 
of the same species. Nearly fifty years ago Chamisso ascertain 
that the solitary Salpa gives birth to a chain, and that each of 
the chain-salpe in turn gives birth to a single solitary Salpa ; 
and to this phenomenon he gave the name of “alternation of 
generations.” At about the same time he published his famous 
story of Peter Schlemihl, who, for an inexhaustible purses sol 
his shadow to the devil, and, through lack of this important 2% 
pendage to his body, became involved in numerous entertaining 
misfortunes and vexations which his money was powerless “f 
prevent. At this time nothing was known in regard to the won 
