CONTRIBUTIONS TO THE STUDY OF VARIATION. 61 



opercular processes of Serpula are identical with the simple antennae 

 of Sabella. 



Now in the latter animal, we have evidence (B, ut sup.) that in 

 the course of development, the branchial filaments are at first simple 

 and antenna-like, so as we know by- A that the branchiae and the 

 antennae are in reality homologous — having the same innervation — 

 we may conclude that the antennae in Sabella are the unaltered 

 first dorsal pair of organs that result from the splitting of the 

 wing-like processes of the very young larva, while the other two 

 lobes of this winged organ and the others that arise by budding on 

 the ventral side, become bipinnate and function as gills. 



Now in Serpula, I believe the course of evolution was probably 

 as follows : — First came a Sabella-like stage with simple antennae, 

 but probably these were larger proportionately than those now seen 

 in this animal. Next the ends of the antennae became swollen to act as 

 stoppers and to close the tube against intruders. Finally one aborted, 

 and being useless, was reduced to an insignificant filament, which 

 occasionally reverts to its former well developed condition. By this 

 explanation I avoid Fritz Miiller's supposition of a Filigrana and 

 Protida ancestry, and this seems to me advisable, as there appears 

 no record, save Miiller's own very incomplete and vague observation, 

 of the opercular filament ever being pinnate, either in variation or in 

 larval development. 



I incline to the belief that Filigrana, Protula, &c, had inde- 

 pendent evolution to Serpula ; probably by the development of all 

 the processes of larval "wings" into pinnate gills, and the subsequent 

 formation of false opercula on the summit of one or more of these. 



References : — 



1. Johnston, Geo. — Gat. of Worms in Br. Mus., 1865, 



p. 270, PI. xx, Fig. 6—7. 



2. Quatrefages, A. de. — Hist. Nat. des Anneles, 1865, Vol. ii, 



p. 401, also PI. iii. 



3. Pruvot. — Archiv. de Zool. Experim. et Gen., 2nd 



Series, t. iii, 1885. 



4. Claus, C. —TraiU de Zoologie, 1884, p. 601. 



5. Muller, Fritz. — Facts for Darwin, 1869, p. 113. 



Explanation of Pl. V, Figs. 1 — 15. 



Serpula pectinata. 



Fig. 1. — Normal antenna ; Figs. 2 — 10. — Abnormal forms of the 

 antenna. 



