THE ANCESTRY OF THE VERTEBRATES 633 



Some supporters of the annelid hypothesis compare the external gills 

 of fishes and amphibia with the filamentous gills attached to the parapodia 

 of annelids. To convert these into internal gills, it is assumed that they 

 wander into the openings of the nephridia. Most suggestions of this 

 sort are couched in Lamarckian terms which make little appeal to the 

 morphologists of this generation. 



Dohrn ('75) and Marshall ('79) regard the olfactory pits of vertebrates 

 as the anteriormost pair of pre-oral gill-pouches, and hence unlike any 

 structures in annehds. Balfour ('81) and Gegenbaur ('87), however, 

 opposed the assumption that olfactory pits are gill-pouches, on the ground 

 that olfactory pits are ectodermal, while gill-pouches are endodermal. 

 Advocates of the annelid hx-pothesis are inclined to compare the olfactory 

 pits of vertebrates with a pair of ciliated pits located in the dorsal half 

 of the prostomium of annelids. With a reversal of surfaces, the pits 

 would be brought into their definitive position on the ventral side of the 

 snout of fishes. 



Eisig ('87) believed that the lateral line organs of vertebrates a'C 

 represented in annelids by certain sense-buds found in capitellid worms. 

 These sense-organs are metamerically arranged in annelids as are the 

 lateral line organs of teleost embryos. Their structure is also similar. 

 Their innerv^ation, however, is different, for lateral line organs are inner- 

 vated by cranial nerves which extend through many metameres, the 

 segmental sense organs of annelids by segmental nerves. To explain this 

 difference, it is assumed that a collector nerve, which originally connected 

 the segmental nerves of the trunk, has become the lateralis branch 

 of the vagus nerve. The epibranchial ganglia of vertebrates are supposed 

 to be comparable with the lateral or parapodial ganglia of annelids, 

 which have been preserved only in the head region of vertebrates. 



The intestinal muscles of annelids are arranged like those of chordates 

 in circular and longitudinal layers. The resemblances of the somatic 

 muscles is not so striking. Except possibly in the gill region, the circular 

 muscles of annelids are lacking in chordates. Annelids have longitudinal 

 metameric muscles on both dorsal and ventral sides of the body. In the 

 trunk region of chordates, muscles are primarily lacking in the ventral 

 half of the body; but are secondarily supplied by migration from the 

 dorsal side. Acceptance of the annelid theory, therefore, makes it 

 necessary to assume that the dorsal muscles of annelids have disappeared 

 in the course of phylogenesis. 



Some morphologists regard the transient neuromeres of vertebrate 

 embryos as evidence in favor of the annelid theory, and interpret neuro- 

 meres as remnants of the segmental ganglia of the annelid nerve-cord. 

 It has, however, not been demonstrated that neuromeres are metameric 

 structures. Serial repetition of structures does not demonstrate that 



