ORIGIN AND STRUCTURE OF THE HEAD 59 



may be asked if it is advisable or of any use to carry any 

 further our speculations in this direction. Yet I will do so and 

 will venture to draw some more conclusions which, whether 

 they are confirmed or disproved in the future, in any case 

 will have been of some use as a stimulus to further research. 



Lahral ganglia of Annelids homologous to spinal ganglia 

 of Vertebrates? — In Annelids every lateral sense-organ is 

 closely connected with a ganglion, the lateral ganglion, which 

 envelops the elements of the ciliated area like a cap. These 

 latcal ganglia are considered by ElSIG (1887, p. 517) to 

 be homologous to the parapodial ganglia of other Annelids, 

 which ganglia, in agreement with this view, are absent in 

 Capitellids and Polyophthalmids, which latter possess the 

 highest developed lateral organs. The development of the 

 parapodial ganglia in Lopadorhynchus has been described and 

 figured by Kleinenberg (1886 p. 112). They originate as 

 proliferations from the ectoderm, not far from but independent 

 of the ventral ganglia, and separated from the latter by 

 the ventral muscle plate. Now KLEINENBERG (I.e. p. 220) 

 had supposed that the spinal ganglia of Vertebrates cor- 

 respond to the parapodial ganglia of Annelids, and this 

 view is joined by EISIG (1887, p. 542). "Und auch die 

 Frage, warum denn erstere Ganglien bei den Vertebraten 

 nicht mehr so wie diejenigen der Hirnnerven zu der Haut, 

 respective den Seitenorganen ontogenetische Beziehungen- 

 aufweisen, lasst sich beantworten. Derselbe durch die Con- 

 centrirung des Kopfes oder Gehirnes hervorgerufene Process, 

 der an den ubrigen Bestandtheilen des Seitenorgansyste- 

 mes so tiefgreifende Veranderungen hervorrief, namlich die 

 Anbahnung eir.er einheitlichen und directen (Gehirn-)Leitung 

 an Stelle der segmentalen, hat auch die ursprunglichen 

 Hautbeziehungen der Seitenorganganglien (Srinalganglien) 

 allmahlich zum Schwinden gebracht" (I.e. p. 543). 



The following, therefore, are, in concise form, the views 

 formulated by ElSIG: the original segmental communications 

 of the lateral organs with the spinal ganglia, the latter being 

 the homologues of the parapodial and lateral ganglia of 

 Annelids, have been preserved only in the head; in the trunk, 

 however, they have been lost and replaced by the collector, 

 the ramus lateralis of the vagus. 



The views resulting from my theory, though agreeing on 

 the whole with those of ElSlG. yet make certain modifica- 

 tions indispensable to bring the latter into agreement with 



