ASCIDIA. 47 
fig. 4, tn.) projects into the mouth cavity and forms a 
sensitive sieve or strainer, through which all the sea-water, 
and its contents, drawn into the branchial sac has to 
pass. 
The tentacles are simple and tapering in Asczdia, but 
in many other Ascidians they are compound and may be 
very elaborately branched (e.g., in Cynthia and Molgula). 
They are in many cases of different sizes arranged alter- 
nately or with some marked symmetry. In Asceidia 
mentula there are usually from 70 to nearly 100 tentacles, 
of which one-third, say 20 to 30, are much larger than the 
rest. The rule is for two, occasionally three, much smaller 
tentacles to be placed between each pair of larger ones 
visible to the eye. Fig. 7 on Pl. Il. shows, what is 
sometimes found, three orders of tentacles placed symmet- 
rically, the middle one of each group of three smaller ones 
being longer than its two neighbours. In Ascidia virginea 
also the tentacles are very numerous, nearly 100, and are 
of two sizes placed alternately. 
Each tentacle is practically an ingrowth of the connective 
tissue of the body-wall, covered by the epithelial lining of 
the front of the alimentary canal. It has consequently a 
connective tissue core containing muscle fibres and nerves, 
and one or more blood lacune continuous with those of 
the body-wall. The delicate epithelium with which it 
is covered contains some simple sensory cells. These 
tentacles not only act mechanically in preventing large 
objects from entering, but are also sensitive like the lobes 
of the apertures, although only scattered sensory cells and 
no specially differentiated sense-organs are present. 
Behind the tentacles hes the plain or papillated pre- 
branchial zone (Pl. II., fig. 7, p.br.z.) bounded behind by 
a pair of parallel and closely placed ciliated ridges with a 
groove between —the peripharyngeal bands (p.p.b.) — 
