of the Tunicata and tlie Polyzoa. 



281 



(Fig. 5) ; the gill in Doliolam then plainly consists of a great branchial sinus, car- 

 rying its respiratory bars on each side as in Clavelina, but differing from the 

 disposition of parts in the latter genus by having its posterior extremity pro- 

 longed downwards till it reaches the inferior wall of the thoracic chamber, along 

 which it then runs forwards parallel to the superior portion. The mouth per- 

 forates this inferior prolongation of the sinus, and thus becomes related to the 

 sinus and its bars exactly as the mouth in the Fm. 5. 



Polyzoa is to the lophophore and tentacula in 

 these. Savignt informs us, that the mouth opens 

 between the inferior and superior gill in the Salpce 

 examined by him; but it is not easy to determine 

 from his description whether these portions are 

 directly continuous, as in DoUolum. In Doliolum, 

 moreover, the remote extremities of the branchial 

 bars of one side are quite separate from those of 

 the other, and thus present the open condition 

 which characterizes the tentacular crown in the 

 Polyzoa^ so that the gill oi Doliolum constitutes 

 the exact link by which the branchial sac of the 

 Ascidui! passes immediately into the tentacular 

 crown of the Polyzoa. In Pyrosoma we have also 

 an approach to the open condition of the tentacu- 

 lar crown, for the inferior extremities of the trans- 

 verse bars of one side are separated from those 

 of the other by a considerable space, and, accord- 

 ing to Lesieuk, even become free for some distance 

 from their extremities in the species which he 

 describes. 



The structure and connexions, then, of the ascidian tentacula, together 

 with the modifications actually experienced by the longitudinal and transverse 

 bars in the different forms of Tunicata, and the fact that the tentacular crown 

 in the hippocrepian Polyzoa will admit of a satisfactory explanation in accord- 

 ance alone with the views here taken, afford evidence that the homologues of 



Fig. 5. Ideal lon^tudinal section of Do 

 tiolum, 



a + b, external and middle tunic united 

 c, internal tunic ; rf, d, sinus system 

 c, respiratory orifice ; y, cloacal orifice 

 ff, respiratory bars ; i, i, branchial sinus 

 m, mouth; n, oesophagus; o, stomach 

 p, intestine ; 7, anus ; r, cloaca ; w, ner- 

 vous ganghon. 



