II SERUCLURE——TENTACEES Bie, 45 
= a ~ - = 
branchial siphon into the branchial sac. At the base of the 
siphon, just about the line of junction of the ectoderm of the 
stomodaeum with the endoderm of the mesenteron, is placed a 
circle of simple hair-like tentacles (Fig. 18, ¢z) which stand out at 
right angles to the wall, and more or less completely meet in the 
centre to form a delicate, sensory grid or sieve through which all 
the water entering the body has to pass. These tentacles not only 
act mechanically, but are also sensitive although only scattered 
sensory cells, and no specially differentiated sense-organs are 
found upon them. Behind the tentacles hes the plain, or papil- 
Fic. 19.—Semi-diagrammatic transverse section of Ascidia, passing through the atrial 
aperture, seen from anterior surface, left side uppermost. Af, Atrial aperture ; at.J, 
atrial lobe ; Br.s, branchial sac ; c/, cloaca ; con, connective ; d.b/.s, dorsal blood- 
sinus ; d./, dorsal lamina ; end, endostyle ; g.d, genital ducts ; 7,7’, intestine; 1.2, 
interstigmatie vessel; m, mantle; m.b, muscle-bundles ; ov, ovary; p.dr, peri- 
branchial cavity; 7, rectum ; ren, renal vesicles ; sg, stigmata ; spf, atrial sphincter ; 
t, test ; ¢r, transverse vessel ; ty, typhlosole ; v.4/.s, ventral blood-sinus. 
lated, prebranchial zone (Fig. 21, p.br.z), bounded behind by a 
‘pair of parallel and closely placed ciliated ridges with a groove 
between—the peripharyngeal bands—which encircle the anterior 
end of the branchial sac. 
The branchial sac is very large—much the largest organ of 
the body—and extends almost to the posterior end of the body, 
while the rest of the alimentary canal lies upon its left side. 
The food particles, consisting of microscopic plants and animals, 
are carried in through the branchial aperture by the current of 
water, but most of them do not pass out through the gill-slits to 
the atrium, being entangled in the viscid mucus which passes 
by ciliary action along the groove between the peripharyngeal 
bands. 
