Marvels of the Universe 1043 
waving of which the sea-water is drawn into the inhalent aperture to bathe the branchial sac and 
there yield its free oxygen; thus the Ascidian breathes. Whatever particles of organic matter 
the water contained are then secured by the mucus of the endostyle and passed on to the mouth, 
which is lower down in the body and on the opposite side. The food thus secured—chiefly minute 
floating plants such as diatoms, spores of seaweeds and tiny animal forms—is then passed into the 
stomach, the waste products passing through the intestine into a space between the branchial sac 
and the body wall, known as the atrial cavity; thence they are carried out, together with the 
water which has served for respiratory purposes, by the ex- 
halent aperture. Close by the stomach are the reproductive 
glands, where eggs are formed, which, after fertilization, 
pass through a duct into the atrial cavity, there to hatch 
and develop. ; 
Near the bottom of the sac is the heart, an organ the 
function of which is very remarkable, and to which there is 
nothing analogous in any other living thing. Instead of 
sending the blood coursing constantly in one direction, as 
do the hearts of all other animals, the Ascidian’s heart 
throws from twenty to forty consecutive currents in one 
direction, then stops for a few seconds and starts again, 
sending the same number of currents in the opposite direc- 
tion, and so on constantly. In some of the small trans- 
parent forms, such as Clavellina (page 1044), this curious 
operation can be watched by means of an ordinary pocket- 
magnifier. 
Thus much for the structure and functions of the adult, 
which a glance at the diagram on page 1047 will make clear 
and which will be seen to be simplicity itself. But now 
comes another phase in the life-history of these animals. 
The eggs which have passed into the atrial cavity develop 
into young. But these young are not like the parent. 
Instead of being simple bag-like organisms, each little 
Ascidian is to all intents and purposes a young vertebrate. 
They vary a little in outward form, according to species, 
but they are all on the same general plan, which is approxi- 
f 5 [By J. Sinel. 
mately that of a tadpole. It is in this larval or tadpole COMPOUND SEA SQUIRTS. 
stage that the structural details which give high rank to Theunumeronsrepecicerofabetrellisparcvalevays 
these animals are seen. found associated in rosette-like “‘systems’”’ im- 
mersed in a leathery layer on the rocks or sea- 
Referring to the diagram on page 1046, we see that in weeds. There are a number of ‘‘systems” in 
(ing tail Ghar ts @ jotnied mod or mouodaord, amél & Gio) Hee Uwe Gecdic Ae Gras axe: 
nerve, foreshadowings of the backbone and spinal cord of the vertebrates. Also a rudimentary 
brain, an optic nerve and an eye. 
Now these little tadpoles, after being launched into the outer world through the exhalent aperture 
of the parent, lead an independent, active, roving life, and seem full of promise. But this is only for 
a few days; lassitude seems to overtake them, and soon they settle down upon a rock, a stone or a 
seaweed. They alight upon their heads and by means of the three little suckers that project from 
this, they make fast to whatever object they have alighted on and settle down for life. Little by 
little the tail, the incipient backbone and spinal cord, the brain and the eye, are absorbed and 
disappear. Then, of every grace and every virtue shorn, the active little tadpole becomes a mere 
Ascidian or Sea Squirt. 
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