which the heart is developed in man and all the other vertebrates. In
the lower muscular wall of the gut we find a weal-like thickening, a
solid, spindle-shaped string of cells, which becomes hollow in the
centre; it begins to contract in different directions, now forward and
now backward, as is the case with the adult Ascidia. In this way the
sanguineous fluid accumulated in the hollow muscular tube is driven in
alternate directions into the blood-vessels, which develop at both
ends of the cardiac tube. One principal vessel runs along the dorsal
side of the gut, another along its ventral side. The former
corresponds to the aorta and the dorsal vessel in the worms. The other
corresponds to the subintestinal vein and the ventral vessel of the
worms.

With the formation of these organs the progressive development of the
Ascidia comes to an end, and degeneration sets in. The free-swimming
larva sinks to the floor of the sea, abandons its locomotive habits,
and attaches itself to stones, marine plants, mussel-shells, corals,
and other objects; this is done with the part of the body that was
foremost in movement. The attachment is effected by a number of
out-growths, usually three, which can be seen even in the
free-swimming larva. The tail is lost, as there is no further use for
it. It undergoes a fatty degeneration, and disappears with the chorda
dorsalis. The tailless body changes into an unshapely tube, and, by
the atrophy of some parts and the modification of others, gradually
assumes the appearance we have already described.

(FIGURE 2.225. An Appendicaria (Copelata), seen from the left. m
mouth, k branchial gut, o gullet, v stomach, a anus, n brain (ganglion
above the gullet), g auditory vesicle, f ciliated groove under the
gills, h heart, t testicles, e ovary, c chorda, s tail.)