1042 Marvels of the Universe 
leathery coat or tunic, and they are also known as Ascidians, because, in outward form, they 
bear some resemblance to the old-time skin wine-bottle, or Askos. 
Two thousand two hundred years ago, in his great work, “ History of Animals,’ the naturalist 
Aristotle wrote about the Ascidians, and naturalists have been writing about them almost ever 
since, but it is only in these latter days that their place in nature has been discovered. Until a few 
years ago they were thought to be somehow related to the molluscs, and were classed as ‘‘ Mollus- 
coidea.’’ But modern science has unravelled their life-history and through embryology has discovered 
their place in nature. 
These lowly-looking animals that, to the ordinary observer, are just leathery bags filled with 
something soft, are now shown to hold a kind of cousinhood with the highest of nature’s children, 
the vertebrates. When this fact was first discovered it was supposed that they represented an 
ancestral vertebrate form; but it is now held that such is not the case, but that they are just 
unsuccessful competitors in the evolutionary march. Their ancestors in the far past had started 
on the vertebrate road and had made good progress when they took a wrong turn. This much 
embryology, which is an epitome of the history of the race, clearly shows. 
Immensely varied in shape, in size, and in habit as are these Sea Squirts, they are all built on 
one plan, which is as follows: In adult life they consist of a bounding membrane more or less 
strengthened with a leathery or even horny tunic. Within this is a branchial sac which acts as the 
gills do ina fish. At one side of this isa grooved, rod-like organ, termed the endostyle, which secretes 
an adhesive mucus and the function of which we shall see ina moment. At the upper part of the 
body is an opening termed the inhalent aperture, and at one side is another, termed the exhalent 
aperture. The linings of these are clothed with fine vibratile microscopic threads or cilia, by the 
Photo by] (J. Sinel. 
RED CURRANT SEA SQUIRTS. 
A natural group upon the rock is here shown one and a half times the actual size. It is one of the commonest forms of 
these interesting animals, 
