92 ASCIDIANS CHAP. 
apertures (Fig. 57, ¢’). There is only a single layer of adult 
ascidiozooids in the wall of the Pyrosoma colony, as all the fully 
developed ascidiozooids are placed with their antero-posterior 
axes at right angles to the surface and communicate by their 
atrial apertures with the central cavity (Fig. 56, C). Their 
dorsal surfaces are turned towards the open end of the colony, 
and the buds are given off from their ventral edges (Fig. 57). 
Anatomy.—The more important points in the structure of 
the ascidiozooid of Pyrosoma are shown in Fig. 57. <A cirele of 
Fic. 57.— Ascidiozooid of 
Pyrosoma from the right 
side. a, Anus; At, atrial 
aperture ;  at.m, atrial 
muscles ; Br, branchial 
aperture ; b7.s, branchial 
sac ; cl, cloaca; d.l, dor- 
sal lamina; d.¢, dorsal 
tubercle ; ec, ectoderm ; 
en, endoderm ; end, endo- 
styles. 2%.) heartistaaeo; 
luminous organ; mes, 
mass of mesoderm cells ; 
m.f, muscle fibre; 9, 
nerve-ganglion ; oes, 0eso- 
phagus ; sg, stigmata; st, 
stomach ; stol, stolon ; 
t, test; ¢’, projection of 
test near branchial aper- 
ture ; tes, testis ; tm, ten- 
tacle ; 1, 2, 3, buds. 
tentacles, of which one, placed ventrally (tn), is larger than the 
rest, is found just inside the circular branchial aperture. From 
this point a wide cavity, with a few circularly placed muscle- 
bands running round its walls, leads back to the large branchial 
sac (br.s.), which occupies the greater part of the body. The 
large stigmata are elongated transversely (dorso-ventrally), and 
are crossed by internal longitudinal bars running antero-posteriorly. 
The dorsal lamina is represented by a series of eight or ten 
