CONTRIBUTION'S TO THE STUDY OF VARIATION. 57 



II. Duplication and Origin of the operculum in Serpula. 



It is of frequent observation that certain animals are charac- 

 terised by more or less mutability, either as entire organisms or else 

 in some particular organ. Such mutable species are, however, greatly 

 in the minority — the vast majority being so remarkably stable in all 

 save trivial degrees, as really to be cause for wonderment, considering 

 the diversity of environment that most species must encounter in 

 life's struggle. Accordingly the forms exhibiting marked mutability, 

 are of the greater interest. These variations fall naturally into three 

 categories ; a, where the mutation is due to an atavistic cause, i.e., 

 recalling some phase in the past history of the race to which the 

 animal belongs ; b, where it is spontaneous or original, and usually 

 limited either to one individual or to a few nearly related ones — a 

 variation often pathological ; lastly, c, where the change is due 

 directly to altered environment and altered habits — a change which 

 will probably become fixed, and therefore producing a distinct and 

 permanent variety — liable to become sufficiently stable to constitute 

 finally a new species. 



The following instances fall, I believe, to be explained by the 

 first of these causes : — 



I. Serpula.* Having recently occasion to examine a large 

 number of that most beautiful Serpulid, S. (Hydroides) pectinata, 

 Philippi, to my surprise, I found 25 °/ o of the examples — which be it 

 noted were not all from the same cluster of tubes or vermidom, but 

 from several — affected in more or less degree with abnormality in the 

 antenna.-f- The proportion was very constant ; out of two large 

 batches, the variation was not more than 1'6 % viz., 8 abnormal out 

 of 32, and 14 out of 60. Others I have examined since, have shown 

 a like proportion. 



In the present note, the two antennas are distinguished for con- 

 venience under the terms respectively of antenna and of operculum : 

 the former term being applied to the process which, while homologous 

 to the long filament modified to form an operculum, is normal short 



* For those unacquainted with the Serpulinse, I may here explain that they are 

 those tube-building worms, whose sinuous limy tubes are frequently seen upon old 

 and sea-worn shells — oysters, scallops and the like. Attached to the head in these 

 animals, are two half circlets of delicate bipinnate plumes, useful as breathing (gills) 

 and touch organs, while close to the first gill-filament on either side on the dorsal 

 aspect is a non-pinnate organ — on one side, long stalked and stopper-like, the oper- 

 culum ; on the other, short and weak and nearly aborted. Serpulids have the power 

 of withdrawing entirely within their tubes, and of stopping the entrance with the 

 plug-like operculum. 



t I adopt here Quatrefages' nomenclature, viz., call the appendages that belong 

 to the prostomium, antennse ; and reserve the term tentacles for the appendages 

 of the peristomium. (Peristomium;= mouth somite; prostomium = region anterior 

 to such). 



