60 ASCIDIANS ‘CHAP. 
has formed the beginning of the muscular body-wall, the con- 
nective tissue around the organs, and the blood; the endostyle has 
developed as a thick-walled groove along the ventral edge of the 
pharynx, which has become the branchial sac; and the peri- 
cardial sac and its invagination the heart have formed in the 
mesoblast between the endostyle and stomach. The “ epicardiac 
tubes ” grow out from the posterior end of the endostyle to join the 
pericardium. They play an important part in the formation of 
Fic. 26.—Metamorphosis of an Ascidian. A, free-swimming tailed larva; B, the 
metamorphosis—larva attached ; C, tail and nervous system of larva degenerating ; 
D, further degeneration and metamorphosis of larva into E, the young fixed Ascidian. 
at, Atrial invagination ; ch, notochord ; hy, hypoblast cells ; 7, intestine ; m, mouth ; 
mes, mesenteron ; 7.c, neural canal ; n.v, neural vesicle with sense-organs. (Modified 
from Kowalevsky and others.) 
buds in the colonial Tunicata. The heart acquires a connexion 
with blastocoelic blood-spaces at its two ends. The heart 
and pericardium show the same relations in Tunicata as in 
Enteropneusta, but it is very doubtful whether these organs are 
genetically related to the Vertebrate heart. 
The unpaired optic organ in the cerebral vesicle when 
fully formed has a retina, pigment layer, lens and cornea; while 
the ventral median organ is a large, spherical, partially- 
pigmented otolith attached by delicate hair-like processes to 
the summit of a hollow “ crista acustica” (Fig. 26, A). After 
a few hours, or at most a day or so, the larva attaches itself 
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