. The Encyclopaedia Britannica; ... A dictionary of arts, sciences and general literature . oticedby Hancock, communicates anteriorly,as Ussoff {2j) pointed out, by meansof a narrow duct with the front ofthe branchial sac (pharynx). Theopening of the duct is enlarged toform a funnel-shaped cavity, whichmay be folded upon itself, convoluted,or even broken up into a number ofsmaller ? openings, so as to form acomplicated projection, called thedorsal tubercle, situated in the dorsalpart of the prebranchial zone (fig. 7).The dorsal tubercle in A. mentula issomewhat horse-shoe-shaped (fig. 8);it va
. The Encyclopaedia Britannica; ... A dictionary of arts, sciences and general literature . oticedby Hancock, communicates anteriorly,as Ussoff {2j) pointed out, by meansof a narrow duct with the front ofthe branchial sac (pharynx). Theopening of the duct is enlarged toform a funnel-shaped cavity, whichmay be folded upon itself, convoluted,or even broken up into a number ofsmaller ? openings, so as to form acomplicated projection, called thedorsal tubercle, situated in the dorsalpart of the prebranchial zone (fig. 7).The dorsal tubercle in A. mentula issomewhat horse-shoe-shaped (fig. 8);it varies in form in most Ascidiansaccording to the genus and species,and in some cases in the individualalso. Possibly, besides being the fio,opening of the duct from the sub-neural gland, it may be a sense-organfor testing the quality of the waterentering the branchial sac. The single elongated ganglion inthe median dorsal line of the mantlebetween the branchial and atrial si-phons is the only nerve-centre in A. mentula and most otherTunicata. It is the degenerate remains of the anterior. 7.— Diagrammatic sec-tion tlirough anterior dor-sal part of A. mentula,showing the reations ofthe nerve cangLion, sub-neural gland, &c. Lett^ir-ing a3 for fig. 4; n, nerve ;n, myelon ; pp, peripha-ryngeal band; sgU sub-neural gland ; eyd, itaduct; C, test liningbrancli-ial siplion. (Original.)
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