VARIANTS OF ANNELID THEORY 285 



shown by Eisig, and by Spengel, to be the neurilemmar sheaths 

 of thick nerve fibres which had in many cases degenerated. 

 The view that the content of the neurochordal tubes was 

 nervous in nature was first promulgated by Leydig in 1864. 



Much difference of opinion reigned as to the true homo- 

 logies of the brain and mouth of Annelids and Vertebrates. 



o 



Beard l and others got over the difficulty of the haemal 

 position of the cerebral ganglion in Annelids by supposing 

 that it degenerated and disappeared altogether in the 

 Annelidan ancestor of Vertebrates, and that accordingly 

 it had no homologue in the Vertebrate nervous system. 

 Beard put forward also the ingenious theory that the hypo- 

 physis represents the old Annelidan mouth. 



Van Beneden and Julin 2 assumed that in the ancestors of 

 Vertebrates the oesophagus shifted forward between the still 

 unconnected lobes of the brain to open on the haemal surface. 



The fundamental assumption of the Annelid theory, that 

 dorsal and ventral surfaces are morphologically interchange- 

 able, seemed rather bold to many zoologists, and Gegenbaur 3 

 voiced a common opinion when he rejected as unscientific 

 the comparison of the ventral nerve cord of Articulates with 

 the dorsal nervous system of Vertebrates. 



The Balanoglossus theory of Vertebrate descent also 

 belongs, at least in its first form, to the earlier group of 

 evolutionary speculations The gill-slits of Balanoglossus 

 were discovered by Kowalevsky as early as i866. 4 Tornana 

 was discovered by J. M tiller in 1850, but by him considered 

 an Asterid larva ; its true nature as the larva of Balanoglossus 

 was made out by Metschnikoff in 1870, who also remarked 

 upon its extraordinary likeness to the larvae of Echinoderms. 5 



1 "The Old Mouth and the New," Anat. A/is., iii., 1888. Nature, 

 xxxix., 1889. 



2 " Recherches sur la Morphologie des Tuniciers," Arch, dc Biol. t 

 vi., 1887. 



3 " Die Stellung u. Bedeutung der Morphologic,'' Morph. Jahrb., i., 

 pp. 1-19, 1876. 



4 "Anatomic des Balanoglossus,'' Mem. Acad. Sci. St Pctersbourg 

 (Petrograd), (7), x., 1866. 



5 Zcit.f. wiss. Zool., xx., 1870. For a recent view of the relation of 

 the Enteropneusta to the Echinoderma, see J. F. Gemmill, Phil. Trans. 

 B., ccv., pp. 213-94, 1914. 



