ON PTYOHODERA FLAVA. 179 



To return toPtychodera flava, the formation of a tem- 

 porary atrial cavity round the branchial sac by the mutual 

 approximation of the genital pleura is a most striking fact. 

 Spengel calls attention to this, and rightly urges that the 

 peripharyngeal cavity so formed is more readily comparable 

 to the atrium of Araphioxus than anything else that has been 

 suggested. He then goes on to add, however, that in his 

 opinion it is nothing but an entirely superficial resemblance. 



Nevertheless it is a real resemblance. The branchial sac 

 being dorsally placed in accordance with the principle above 

 referred to, the peribranchial cavity must be also dorsal in 

 Ptychodera. Presumably there can be no doubt that there is 

 a general homology between the atrium of the Ascidians and 

 that of Amphioxus; and yet in the former it is a dorsal 

 structure (except in the free-swimming Appendicularise), and 

 in the latter ventral. 



With regard to the synapticula or cross-bars in the branchial 

 skeleton, Spengel (loc. cit., p. 725) draws attention to the fact 

 that they are not present in the genera Glandiceps and Balano- 

 glossus, but are present in Ptychodera and Schizocardium, 

 which, he says, are probably younger phylogenetically. But 

 it is very much open to doubt whether Ptychodera is phylo- 

 genetically younger than the other genera of the Entero- 

 pneusta. 



On page 357 of his beautiful monograph Spengel gives a 

 list of what he regards as signs of a primitive organisation in 

 the group. These are open to the criticism that they are, 

 without exception, all negative properties, — the lack of this, 

 that, and the other. 



Then with regard to Ptychodera he says (p. 358), '' Als die 

 hochste Porm erweist sich endlich Ptychodera.'^ For my part, 

 I deny this, and oppose the view on the following grounds. 



In the first place, the positive fact of the diffuse arrange- 

 ment of the gonads, which is a characteristic of P. flava and 

 of the other species belonging to the sub-genus Clilamydo- 

 thorax, bears all the marks of an archaic type. 



Secondly, it seems only reasonable to suppose that the 



