RHODOSOMA HUXLEYI. 711 



More recently Dr Sluiter has accepted my observations as affording an explanation 

 of the absence of the branchial sac in Styeloides, and he thinks that this generic name 

 should lapse in favour of Polycarpa 1 . 



Another notewj.thy Ascidian which I found in Blanche Bay on a reef-patch off 

 Baravon, is called Rhodosoma huxleyi, and was described under the name Per a huxleyi 

 in 1862 by Dr J. D. Macdonald, who obtained his material from the Bellona Reef 2 . 

 The peculiarity of Rhodosoma is this, that the upper portion of the test fits like 

 a lid upon the rest of the tunicary and can be raised and depressed like the valve 

 of a lamellibranchiate Mollusc. Compared with those bivalve Molluscs where one 

 valve is fixed to the substratum, the action of the lid of Rhodosoma which protects 

 the siphons bears a striking analogy to that of the freely hinged valve of the former. 

 It is, in fact, a good example of parallel evolution, a phenomenon of wide occurrence 

 and profound significance, but not always easy to recognise. By what may be termed 

 a singular coincidence, the late distinguished Professor H. de Lacaze-Duthiers 3 , who was 

 one of the first to publish a full account of Rhodosoma, was at great pains to demon- 

 strate the supposed molluscoid affinities of the Ascidians, and the discovery of a species 

 of this peculiar valvate genus confirmed him in what most naturalists will now admit 

 to have been an interesting and instructive error. 



Fig. 11. Rhodosoma huxleyi ; front view with operculum raised ; side-view with lid closed. 

 (Semi-diagrammatic, partly after Macdonald.) 



Finally there is a third Ascidian which is abundant in Blanche Bay and 

 neighbouring localities (New Guinea, etc.) and claims notice on account of its size and 



1 Sluiter, C. Ph., " Tunicaten von Sud-At'rika. " Zool. Jahrb. Syst. xi., 1898, p. 5. It is not known 

 whether the viscera can be regenerated, but Dr Sluiter thinks he has found indications of a regeneration of 

 the endostyle in Styeloides abranchiata. 



2 Macdonald, J. D., "On a new genus of Tunicata (Pera) from the Bellona reefs." J. Linn. Soc. Zool, 

 yi. p. 78, 1862. I am not acquainted with any later record of this species. 



3 Lacaze-Duthiers, H. de, " Sur un genre nouveau d'Ascidien (Clievreulius)." Ann. Sci. Nat. (5) iv. 1865, 

 p. 293. The name Rhodosoma, which takes priority, was introduced by Ehreuberg in 1828. 



