EVOLUTION OF ORGANS IN THE CHORD ATA. 275 



the postanal gut disappeared ; the degeneration of the postanal 

 gut is actually repeated in ontogeny. The contraction of the 

 ventral part of the tail thus brought about caused the series of 

 dorsal parapodia behind the anus to coalesce in the same manner 

 as the ventral parapodia, and thus the median anal fin was 

 produced. The prseanal dorsal parapodia were never approxi- 

 mated laterally, but partly disappeared, partly coalesced longi- 

 tudinally to form the existing pelvic and pectoral fins. The 

 fins therefore have nothing to do with gills, either in the way 

 supposed in Gegenbaur's Archipterygium theory, or in the 

 way originally suggested by Dohrn in the ' Ursprung der 

 Wirbelthiere.^ In the theory now taught by Dohrn the 

 metameric external gills of Annelids are left out of considera- 

 tion ; the ancestor, it is to be presumed, had none. The facts 

 on which the theory is based, and which are important results 

 of investigation however explained, are as follows : — The mus- 

 culature of the pectoral fin is derived in embryos of Elasmo 

 branchs (Pristiurus) from a series of muscle buds separated 

 from the ventral end of each myotome. Each bud divides 

 into four pieces, two above and two below. The same is true 

 of the pelvic fin. That these fins cannot be serially homo- 

 logous with any parts of the gill arches is proved by the fact 

 that the musculature of the gill arches is derived from the 

 head cavities, and these are ventral to the myotomes. So also 

 the gill cartilages are not homodynamous (serially homologous) 

 with the ribs, for the ribs are between the myotomes, the series 

 of which is continued anteriorly above the gill arches. A 

 large number of myotomes contribute to form each fin. Behind 

 the anus on each side muscle buds are given off from the 

 ventral ends of the myotomes ; these are serially homologous 

 with those already described, and in all probability, although 

 the transformation was not traced, they form the musculature 

 of the anal fin. The musculature of the dorsal fins arises from 

 buds given off dorsally exactly as those belonging to the paired 

 fins are given off ventrally. The fin rays in the dorsal fins 

 arise as median cartilaginous rays, at first quite uucounected 

 with any other part of the skeleton. One would have expected 



