HEKATEROBBANCHUS SHRUBSOLII. 191 



from which the ancestors of the two families (Spionidse and 

 Serpulidse) have been derived. It would appear to be nearest 

 to the tribe Amphicoridae of the family Serpulidse. 



In order to grant that Hekaterobranchus does thus connect 

 the two families we must follow Meyer in his recent and 

 very interesting paper on the homologies of the branchiae of the 

 Serpulidse ('Mitth. Zool. St. v. Neapel/ vol. viii, 1888), and 

 grant that the cephalic branchiae of the Serpulidae are develop- 

 ments of the cephalic tentacles of the Spionidse. The branchiae 

 of the Serpulids have been clearly shown to be prostomial 

 organs, both in development and in innervation. The ten- 

 tacles of the Spionids are, according to Leuchart,^ and Leuck- 

 art and Pagenstecher," prostomial in origin. According to 

 Jacobi (' Polydoren der kieler Bucht, Inaugural Dissertation 

 zur Erlangung der Doctorwiirde, 1883, pi. ii, fig. 27, p. 23) 

 they are innervated from the prostomial ganglion. So far we 

 should have no difficulty in deriving the complex branchiae 

 of Serpulids from the simple tentacles of Spionids. Were 

 the simple remaining pair of dorsal branchiae of Hekatero- 

 branchus to disappear, still more work would be thrown on 

 the cephalic tentacles ; and an organ with these important 

 functions (respiration, prehension, and tactile sensibility) local- 

 ised in it would be subject to great variation, and would 

 consequently develop rapidly. Thus we should find these 

 organs first multiplying, but remaining simple as in Haplo- 

 branchus and Manayunkia/ then each becoming more com- 

 plex and giving off secondary rays, as in Fabricia and Amphi- 



^ 'Arch. f. Naturg.,' 21st yr., p. 63, &c., pi. ii, Leuckart says they 

 develop on either side of the " Kopfhocker " (? = prostomial crest), between 

 it and the enlarged "upper lip." This "upper lip," he says, marks the 

 boundary between the pro- and peri-stomium ; and where the tentacles join it 

 there is a group of long cilia on either side, probably representing the remains 

 of the cephalotroch of the larva. 



2 ' Miiller's Archiv,' 1858, pp.610 — 613, pi. xxiii. Here it is said that 

 the middle ciliated band of the larva separates the body into two halves, from 

 the anterior of which the prostomium with the tentacles is formed, and from 

 the posterior of which all the body-segments are formed. 



3 Leidy, ' Proc. Acad. Nat. Sci. Philadelphia,' pp. 204—212, pi. ix. 



