11 



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 Ascidia, 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 Ascidia 

 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 PI. II. 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 lacunae 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 lies the plain or papillated pre- 

 branchial zone (PI. II., fig. l,p.hr.z.) bounded behind by 

 a pair of parallel and closely placed ciliated ridges with a 

 groove between — the peripharyngeal bands (p.p.b.) — 



