Messrs. M. Caullery and F. Mesnil on Spirorbis. 413 



determination of the relation between Mollusca and Vermes. 

 The early notions on this subject extend far back ; it has 

 been set forth in various ways more or less explicit since 

 1844 by Quatrefages, P. J. van Beneden, Carl Vogt, de 

 Lacaze-Duthiers, Morch, Gegenbaur, von Ihering, Giard, 

 Hatschek &c. 



In 1881 (' Les Colonies animales,' p. 631 et seq.} I 

 endeavoured to approach more closely than my predecessors 

 the question which I had already treated in my course at the 

 Museum in 1877, and to define the resemblances of the 

 Gasteropods, considered as the basis of the Mollusca, to the 

 tubicolar Annelids. I remarked in particular (' Les Colonies 

 animales,' p. 640) that the cephalobranchiate Annelids present, 

 like the Gasteropods, *' numerous traces of asymmetry; 

 the Spirographs have one of the cephalic branchiae almost 

 entirely atrophied. Normally in the Serpulidae there should 

 be two opercular appendices, usually but one is developed. 

 The twist of the spiral, so frequent in Gasteropod Molluscs, 

 is found among the Annelids in Spirorbis" This twisting is 

 complicated, according to the interesting researches of 

 MM. Caullery and Mesnil, by an asymmetry external and 

 internal of the most marked kind, and which is equally charac- 

 teristic of Gasteropods. The resemblances of Mollusks to 

 cephalobranchiate Annelids is thus strongly accentuated ; 

 they are, in truth, in part the resemblances of convergence. 

 Is it now permissible to attribute the asymmetry of Spirorbis 

 entirely to adaptive modifications in their anatomy induced 

 by their habitat in the interior of a spiral tube ? There are 

 certain distinctions to be drawn. We have seen already 

 that there are very clear indications of asymmetry in the 

 Serpulidse whose tubes are not spiral; on the other hand, it 

 is the Spirorbis which has constructed its tube, and this tube 

 cannot roll itself into a spiral by reason of an asymmetry 

 already existing, partly at least, in the animal which has 

 produced it. This initial asymmetry is no doubt due to an 

 active cause like that which shows itself in Mollusks (Perrier, 

 1 Trait^ de Zoologie/ p. 2071) ; once the tube is formed, it can 

 accentuate itself by reason of the special conditions of exist- 

 ence it imposes on the animal. But it is essential to remark 

 that these things do not occur here as in the Paguridae, which 

 have adapted for their habitation helicoidal tubes already 

 made. 



