Messrs. M. Caulleiy and F. Mesnil on Spirorbis. 411 



XLIT. — On Spirorbis: Asymmetry of these Annelids and 

 Phylogenic Connexion of Species in the Genus. By MM. 

 Maueice Caullery and Felix Mesnil*. 



We have recently had occasion to examine a great number of 

 Spirorhes from different parts of the globe (Channel, Arctic 

 seas, Mediterranean, Cape Horn, Panama), some twenty 

 species, of which many are new. From a study of these, 

 both from the anatomical and phylogenic point of view, we 

 have arrived at the conclusions which follow. 



The genus Spirorbis belongs to the family Serpulidae ; 

 it is easily recognizable by its calcareous tube, fixed to a 

 support and rolled in a regular spire with contiguous whorls. 

 The thorax of the animal comprises in general three seti- 

 gerous rings, the last two having on each side only a ventral 

 row of unciui. Then follows a long achetous region, en- 

 closing ovules, followed by the abdomen, which is made up 

 of eight to forty segments, of which some contain sperma- 

 tozooids. The prostomium of the animal carries a crowa of 

 feathery branchiae ; one of the branchial rays is modified, 

 deprived of pinnules, and terminated by a calcareous piece — 

 the operculum — which closes the tube when the animal with- 

 draws itself into it. 



We have drawn attention for the first time to the adapta- 

 tions and modifications in the anatomy of Spirorbis induced 

 by its habitat in the interior of a spiral tube. These Annelids 

 have become entirely asymmetrical, of which the facts which 

 follow are the proof : — 



1. The direction of the turn of the spire is constant for a 

 given species. We have called this left-handed in species in 

 which this turning, considered in relation to the free face of 

 the tube, has the same direction as the hands of a watch ; 

 right-handed when the turn is in the reverse direction. Now 

 in the right-handed species the operculum is always borne by 

 the second branchial ray on the right, starting from the medial 

 dorsal line ; in left-handed species it is borne by the second 

 on the left. It is also always found on the concave side of 

 the animal. 



2. The longitudinal muscular fibres are much more deve- 

 loped on the same concave side. 



3. The viscera (digestive tube, ovary) are thrown towards 

 the concave side. 



4. The uncini on the thorax and abdomen are larger and 

 more numerous on the concave side. 



0. The abdomen presents, as a rule, n rows of uncini on 



* From the ' Comptes Rendus,' torn, cxxiv. pp. 48-50. 



