276 



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



Fig. 1. 



Fig. 2. 



Pyrosoma, assumes the heart as indicating the dorsal aspect of the Tunicata;* 

 the cephalic ganghon, however, in those inferior members of the animal king- 

 dom in which the dor- 

 sal and ventral aspects 

 are already indicated 

 by other characters, is 

 invariably placed on 

 the dorsal side of the 

 alimentary canal, and 

 though it be admitted 

 that the almost uni- 

 versal position of the 

 heart among inverte- 

 brate animals is also 

 dorsal ; yet where, as 

 in the Tunicata, we 

 find the ganglion and 

 heart placed on oppo- 

 site sides, the superior 

 importance of the gan- 

 glion will,I think, jus- 



o, stomach ; p^ intestine ; 

 middle tunic ; «, heart ; r, 



9, anus; r, 



Fig. 1. Diagramatic view of Clarelina. 

 Fig. 2. Diagramatic transverse section of 

 Clavelina.^ 



a, external tunic ; b, middle tunic : 

 c, internal tunic ; d d, d, sinus system : 

 e, respiratory orifice ; /, cloacal orifice ; 

 ffyff, transverse respiratory bars ; A, A, longi- 

 tudinal respiratory bars : i, branchial sinus ; 

 k, ft, proper membrane of respiratory sac ; 

 /, languets ; m, mouth ; n, oesophagus ; 

 cloaca ; 5, tentacula ; ty muscular fibres in 



nervous ganglion ; w, gemma. 



tify us in assuming its 

 position as the constant one, and concluding that it is the heart therefore and 

 not the ganglion that has changed place. The only apparent difficulty in as- 

 suming the ganghon as the index of the back results from its not being always 

 obvious that the nervous mass before us is homologous with a true cephalic, 

 or supra-oesophageal ganglion ; there will, however, I think, always be found 

 marks sufficient to decide this point ; we shall subsequently see that the gan- 

 glion, both in the Tunicata and the Polyzoa, undoubtedly contains a supra- 

 cEsophageal element, which from its pre-eminent importance will determine the 



* " Observations upon the Anatomy and Physiology of Salpa and Pyrosoma, together with Re- 

 marks on Doliolum and Appendicularia." By Thomas Henry Huxley. Phil. Trans., 1851. 



t In all the figures accompanying this paper, the same letters are used with the homologous 

 organs. 



