Ascidian. 67 



p. 6c^, as the ' cloaca^ in the Anodon, but which differs from it by 

 being much larger, by not possessing an organ of Bojanus, except in 

 a rudimentary condition, and by not being traversed by any muscle 

 such as the posterior adductor. A wide interspace thus exists here 

 between the branchial sac and the envelopes or tunics placed ex- 

 teriorly to it, to which, however, it is attached continuously along 

 the white band seen along the median ventral line, and known as 

 the ' endostyle / along an encircling zone, placed immediately su- 

 periorly in this preparation to the circlet of tentacles just within the 

 inhalant orifice ; along a line passing from the other end of the 

 endostyle to the neig-hbourhood of the mouth; and lastly at in- 

 tervals in all parts of its circumference except in the cloacal region, 

 by hollow tubular suspenders, which convey blood between its 

 vessels and the visceral and pallial sinuses. The line of the ' endo- 

 style^ is constituted by four folds, the two external being mem- 

 branous, and the two internal of a rigid yellowish substance ; and 

 in this, as also in the relation it holds to a large vessel, the 

 so-called ^thoracic sinus' of Milne-Edwards, passing along it 

 from the heart, and in the more general relations which it holds 

 to the other organs of the animal, it resembles the line of the 

 symphysis of the mantle of the Lamellibranchiata which have the 

 mantle lobes united inferiorly, or the free margin of the mantle 

 lobes of such bivalves as the Anodon (see Preparation 20, p. 61), 

 Along the opposite side of the branchial sac there runs the 'oral 

 lamina,' which in other species, such as AscicUa IntestinaUs, may be 

 represented by a row of ' languettes.' It is connected with the line 

 of the '^ endostyle'' by the encircling zone already spoken of as lying 

 superiorly to the circlet of tentacles. In the interval between this 

 encircling zone and the coronet of tentacles, at the point where the 

 oral lamina becomes continuous with the former of these structures, 

 is seen the ' ciliated sac' or ' anterior tubercle.' Posteriorly to the 

 encircling zone, or 'anterior collar' of Hancock, in the immediate 

 neighbourhood of the anterior tubercle, is seen the single ganglion 

 of the Tunicata, supplying the mantle. This ganglion therefore 

 would seem to be homologous with the branchial or parieto- 

 splanchnic ganglion of the Lamellibranchiata ; and the circlet of 

 tentacles will, by consequence, be seen to be homologous with the 

 tentaculate inhalant portion of the mantle in those animals. 



The ' oral lamina' is seen to be underlaid by a large vessel, the 



F 2 



