of the Tunicata and the Poly zoa. 



277 



Fig. 3. 



Fio. 4. 



dorsal position of the ganglionic mass, even though the latter should also perform 

 functions usually devolving on ganglia situated below the alimentary canal. 



1. Respiratory Sys- 

 tetn. — As it is in the 

 respiratory organs of 

 the two groups that 

 the leading peculiari- 

 ties of their structure 

 are to be found, our' 

 attention must be first 

 directed to this por- 

 tion of the organiza- 

 tion, with the view of 

 determining how far 

 the respiratory appa- 

 ratus in the one has 

 its homologue in the 



other. Now two dis- 

 tinct notions have pre- 

 vailed on this point, 

 some zoologists* main- 

 taining that the res- 



FlG. 3. Diagramatic view of a hippo ■ 

 crepian Polyzoon^T^iT&cXiti). Fig. 4. Dia- 

 gramatic transverse section of a hippocre- 

 pian Pohjzoon. 



a, ectocyst ; 6, endocyst ; c, tentacular 

 sheath ; d, d, d, perigastric space ;/-*-«, 

 external orifice of cell ; g, <?, tentacula : 

 I, lophophore ; A, A, caliciform membrane ; 

 /, oral valve-like organ ; m, mouth ; n, ceso- 

 phagus ; 0, stomach; />, intestine; 9, anus; 



r, cavitj' of tentacular sheath ; t, muscular fibres in enJocyst ; ti, nervous ganglion . 



w, gemma. 



piratory sac of the Ascidian has its representative in the pharynx of the 

 Polyzoon, and that the rudimental tentacula at the orifice of this sac are ho- 

 mologous with the tentacula of the Polyzoon ; while others f assert, that the 

 brancliial sac of the Ascidiie is homologous with the tentacular crown of the 

 Poly zoa, the longitudinal bars of the sac corresponding to the tentacula of the 

 Poly zoa, and the transverse bars becoming extinct. Now, neither of these views 

 appears to me to represent the exact truth, for, while I conceive that the tenta- 

 cular crown of the Polyzoa has undoubtedly its true homologue in the respi- 



* See Dr. A. Farre, " Observations on the Minute Structure of some of the higher Forms of 

 Polypi." Phil. Trans. 1837. 



t See Van Beneden, " Sur les Ascidies Simples." Mem. de I'Acad. Eoy de Belgique, Tome 

 XX., 1847. 



2o2 



