STRUCTURE TENTACLES, ETC. 



4S 



branchial siphon into the branchial sac. At the base of the 

 siphon, just about the line of junction of the ectoderm of the 

 stoinodaeum with the endoderm of the mesenteron, is placed a 

 circle of simple hair-like tentacles (Fig. 18, tn) 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 lies the plain, or papil- 



^9- r-en i' "''■ ,■ 



d.hl'.s. 



m.b.'-'' 



l-'y- m. 



Fig. 19. — Semi-diiigranijiiatio transverse section of Ascidia, passing tlirough the atrial 

 aperture, seen from anterior surface, left side uppermost. At, Atrial aperture ; at.l, 

 atrial lobe ; Br.s, branchial sac ; cl, cloaca ; con, connective ; d.bl.s, dorsal blood- 

 sinus ; d.l, dorsal lamina ; end, endostyle ; g.d, genital ducts ; i, i', intestine ; l.v, 

 interstigmatic vessel ; in, mantle ; m.6, muscle-bundles ; ov, ovary ; jj.6r, peri- 

 branchial cavity ; r, rectum ; ren, renal vesicles ; sg, stigmata ; sph, atrial sphincter ; 

 t, test ; tr, transverse vessel ; ty, typhlosole ; v.bl.s, ventral blood-sinus. 



lated, prebranchial zone (Fig. 21, pM.e), 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. 



