44 



HOMOLOGIES. 



quadranffular meshes thus formed, is perforated bj- an aperture, (the siigmates hrancliiaux of 

 Milne-Edwards),* surrounded by a fringe of \-ibratile cilia. 



It must, moreover, be borne in mind, that the transverse bars open into two great 

 longitudinal canals, the " bi-anchial '' and " thoracic sinuses " of Milne-Edwards, one placed 

 upon the neural, and the other upon the hsemal side of the sac, that they constitute, in fact, 

 a series of communicating channels, passing transversely between these two sinuses, re- 

 cei%-ing the blood from one and pouring it into the other. 



In advocating the homology of the respiratoiy sac of the Ascidian with the tentacular 

 crown of the Polvzoon, some zoologists, among whom must be especialh' mentioned jNI. Van 

 Benedenf, maintain that the longiindinal bars of the sac correspond to the tentacula of the 

 Polvzoon, while the transverse bars in the sac of the Ascidian must be considered as becoming 

 extinct in the tentacular crown of the Polvzoon. Now this view is certainly untenable. I 

 have in another place;}: endeavoured to show that while the respiratory sac of the Ascidian has 

 undoubtedlv its homologue in the tentacular crown of the Polyzoon, it is the transverse and 

 not the longitudinal bars of this sac that are represented by the tentacula of the Polyzoon, 



Fk. 7. 



Fig. 6. Plan of an Ascidian Tunicate— Fig. 7- Plan of an Ascidian Tanicate — 



longitudinal section. transverse section. 



. External tunic, b. Middle tunic, c. Internal tunic, d d d. General sinus system. e. Respiratory 



orifice. /. Cloacal orifice. g. Transverse respiratory bars. h. Longitudinal respiratory bars. 



1. Branchial sinus, k. Thoracic sinus. II. Proper membrane of respiratory sac. m. Languettes 

 n. Month. o. QEsophagus. p. Stomach, q. Intestine, r. Anus. s. Cloaca. t. Tentacula at 



entrance of respiratory chamber, u. Ganghon. r. Heart. 



* See IMilne-Edwards's beautiful memoir, " Sur les Ascidies Composees." 



■j- Van Beneden sur les Ascidies simples. ' Mem. de I'Acad. Roy. de Belgique,' tome xx, 1847. 



i ' Trans. Rov. Irish Acad., Jan. 1852. 



