REPORT ON THE TUNICATA. 147 



Thenumhor of lol)Cs surroumling the liraiicliial and atrial apertures is of cousidcrable 

 importance in classifying the Tunicata, but it is by no means obvious what it is that 



Fig. 2S.— Part of the branchial sac of Ciileolm wyvillii. 

 i.l. internal lougitujinal bars ; mh. mesh; ti: transverse vessels; sjj. spicules. 



determines the exact number of lobes, and why the number should become (-hanged in 

 (jertain groups. 



In the Appendiculariidse, and probably in the Prototunicata, the Protothaliacea, 

 and the Protoascidiacea, the apertures were simply rounded or slightly liilabiate. In 

 Doliolum they have become ten or twelve-Ioljed. in the Balpidse and in Octacnemus they 

 are circular or bilabiate ; finally, in Clavelina, Perophora, and Ilijpohytliius, all simple 

 forms closely allied to the ancestral Protoascidiate occupying the point E. (table, p. 150), 

 the apertures are simply rounded. In Ecteinascidia, however, a slight lobing of the 

 apertures has commenced. In some cases it is not visible, in others six indistinct lobes 

 around the atrial aperture, and seven or eight around the branchial aperture, can be 

 made out. 



In the great majority of the Compound Ascidians derived from the branch F. (table, 

 p. 150) the branchial aperture is provided with six well-marked lobes, while the atrial may 

 be-six-lobed, or Ijilabiate, or simply rounded, or, finally, may be provided with a single 

 very large anteriorly placed lobe known as the atrial languet. In one group of the 

 Polyclinidse, however, consisting of the genera Parasridia, Fragarinm, CircinaUvm, 

 and Morchellioides, the branchial aperture is very distinctly eight-lol)ed. In the 

 remarkable Ccelocormus, again, the branchial apertures are five-lobed. 



In nearly all the Ascidiidae amongst Simple Ascidians the arrangement of the 



