of the Tunicata and the Polyzoa. 289 



lopment of the bud tlirougli the more opaque tissues of these Polyzoa has ren- 

 dered us here deficient in the class of facts now under discussion, and we are 

 not, therefore, yet prepared to institute an actual comparison between the deve- 

 lopment of the branchial membrane in the Ascidice and the caliciform membrane 

 in the hippocrepian Po/y^oa ; so far, however, as our imperfect observations go, 

 the facts are still in accordance with the views of the present paper ; and though 

 we have but little positive evidence to assist us in our conclusions, yet there is 

 not a single observation tending to disprove the position that the branchial 

 membrane of the one, and the caliciform membrane of the other, present in the 

 circumstances of their development the conditions of homologous organs. 



Among the other points of resemblance between the two groups, it is 

 interesting to observe the frequent occurrence among the Tunicata of definite 

 compound phytoidal forms resulting from gemmation, exactly as in the Polyzoa. 



From what has now been stated it must be manifest, that the Tunicata and 

 the Polyzoa are more nearly related to one another than either to any other 

 branch of the animal kingdom ; that they really belong to one and the same 

 great structural type ; and that the differences between them are non-essen- 

 tial modifications of this type, rendered for the most part necessary by the 

 new power superadded upon thePoZ^/zoaof alternately projecting and retracting 

 the respiratory crown and anterior portion of the digestive organs through the 

 external orifice of the cell. 



The homology of the organs in the Tunicata and the Polyzoa, which it has 

 been the object of the present paper to demonstrate, will be rendered more 

 apparent by bringing together the equivalent organs of the two groups in the 

 following two parallel series: 



TUNICATA. POLYZOA. 



External tunic, . . . . = Ectocyst. 



Middle tunic, = Endocyst. 



Internal tunic, = Tentacular sheath. 



Sinus system, = Perigastric space. 



Respiratory orifice, . . ."1 „ ^ -, -i- r- ,, 



^, , . - ) = External orifice ol cell. 



Lloacal orifice, J 



Transverse respiratory bars, = Tentacula. 



