278 Dr. G. J. Allman on the Homology of the Organs 



ratory sac of the Ascidice, I believe that it is to the transverse, and not to the 

 longitudinal bars of this sac that the tentaciila of the Polyzoa are homologous ; 

 and this is a very important distinction, the non-recognition of which has ren- 

 dered all previous attempts at comparison between the tentacular crown of the 

 Polyzoa and the respiratory sac of the Ascidice untenable. 



On this subject much light is thrown by the hippocrepian Polyzoa, or those 

 fresh-water genera which, like Plumatella, have their tentacula arranged on a 

 crescentic " lophophore ;"* and we shall best perceive the relations in question 

 by comparing an ascidian Tunicate witli one of these Polyzoa, a Clavelina (Figs. 

 1 and 2), for example, with a Plumatella (Figs. 3 and 4). In Clavelina, the great 

 "branchial sinus" of Milne-Edwards,! from each side of which the transverse 

 bars or vessels of the respiratory sac are given off, will correspond to the elongated 

 lophophore in Plumatella, and the richly ciliated transverse bars to the ciliated 

 tentacula, while the delicate membranous sac, to the interior of which the respira- 

 tory bars are adherent, and which Milne-Edwards has shown to be perforated in 

 the intervals of these bars by the " respiratory stigmata," will have its homologue 

 in the calyx-like membrane adherent to the base of the tentacular plume in Frede- 

 ricella and the hippocrepian Pt^^y^roa. This correspondence will be rendered more 

 obvious by imagining the branchial sinus to be rotated round its oral extremity in 

 a vertical plane through an angle of 90°, towards the superior or anal side of 

 the Tunicate ; its position from longitudinal will thus be changed to transverse, 

 while the transverse bars will become longitudinal, and the branchial sinus and 

 its bars will then have the same direction as the exserted lophophore and ten- 

 tacula oi Plumatella ; while it is interesting to observe that, during the retracted 

 state of the Polyzoon, the lophophore assumes the normal direction of the bran- 

 chial sinus in the Tunicate. 



That the tentacula of the Polyzoa are not homologous with the unciliated 



* In a Report on the Fresh-Water Poli/zoa, read before the Edinburgh Meeting of the British 

 Association for 1850, I found that our increased knowledge of the structure of the Polyzoa ren- 

 dered it necessary to make some change in the terminology hitherto employed in their description; 

 and the terms used in that Report are also adopted in the present memoir. The Polypide is the 

 retractile portion of the Polyzoon as distinguished from its cell; the Ectocyst is the external tunic 

 of the cell; the Eiulocyst is the internal tunic; the Lophophore is the kind of disc or stage which 

 surrounds the mouth and bears the tentacula. 



t SeeM. Milne-Edwards's beautiful memoir, "Sur les Ascidies Composees." 



