43 



3. 



Homologies of the Polyzoa. 



Before commencing the zoographical portion of the present memoir, there still remains 

 to be considered a subject of great interest to the philosophical zoologist, namely, the exact 

 position and affinities of the Polyzoa in the animal kingdom, a question which admits of much 

 valuable elucidation from the study of the freshwater genera. 



We have seen in the historical sketch already given of the successive steps which resulted 

 in the separation of the Polyzoa from the Polypes, that the molluscan relations of the Polyzoa 

 began at last to be recognised in an obvious resemblance between their organization and tliat 

 of the Ascideae. Guided by this relation, an important step was finally taken by M. Milne- 

 Edwards in distinguishing two primary series in the moUusca, characterised mainly by the 

 relative grade of perfection of the nervous and circulatory systems. One of these, the 

 MoUiiscoida, included the Polyzoa and Tunicata ; the other, the MoUusca proper, embraced all 

 the other members of the molluscan sub-kingdom. The affinities of the Tunicata and Polyzoa 

 being thus fully recognised, it is of importance to know in what these affinities really consist; 

 in other words, to determine the homologous organs in the two groups. 



I had elsewhere* attempted to show what I believed to be the correct view of the relation 

 between the Tunicata and the Polyzoa. Additional opportunities of investigation have 

 suggested a few modifications, but after a careful comparison of all that has been urged on 

 this subject, I am still of the opinion that in its leading points view then taken was the true 

 one. 



The great respiratory sac of the Tunicata is the most striking feature in the structure of 

 this group, and I shall therefore first endeavour to point out where the respiratory sac of the 

 Tunicata is represented in the Polyzoa. 



Now, the opinions entertained on this subject may be classed under two distinct heads. 

 Under the one head it is maintained that the respiratory sac of the Ascidian has its repre- 

 sentative in the pharynx of the Polyzoon, and that the rudimentary tentacula at the orifice of 

 this sac are homologous with the tentacula of the Polyzoon. Under the other head it is 

 asserted that the respiratory sac of the Ascidian is homologous with the tentacular crown of 

 the Polyzoa. I have carefully examined all that has been urged in favour of each of these 

 views, and I believe that the evidence preponderates on the side of considering the respiratory 

 sac of the Ascidian as truly represented by the tentacular crown of the polyzoon, though the exact 

 nature of the homology has not been correctly stated. The reader will call to mind that the 

 respiratory chamber of an Ascidian, consists essentially of a membranous sac, having the inner 

 surface of its walls covered by two sets of tubular bars, one running longitudinally, or from 

 behind forwards, the other transversely, or from the neural to the haemal side, and thus 

 crossing each other at right angles, while the membranous wall of the sac, in every one of the 



* On the Homology of the Organs of the Tuuicata and the Polyzoa. ' Tiansactions of the 

 Royal Irish Acarlemy, vol. xxii, 1852. 



