330 



relations but not used as fixing organs in Amphioxus (head-cavities, 

 praeoral coelom + praeoral pit) and Balanoglossus (proboscis). 



Moreover, von Kupfper's account of the origin of the praemandi- 

 bular cavities in Ammocoetes *) leaves no room for doubting that 

 they are homologous with the head-cavities of Amphioxus ; and in fact, 

 this view has been already maintained in part by van Wijhe (Anat. 

 Anz., Bd. VIII, 1893, No. 5, p. 152 et seq.) and in full by von Kupffer 

 (Entwickelungsgeschichte des Kopfes. Ergebnisse der Anat. und Ent- 

 wick. Merkel und Bonnet, Bd. II, 1893, see p. 524). 



Thus the praeoral lobe of the Protochordata is represented in the 

 craniate Vertebrates by the praemandibular head-cavity (consisting of 

 lateral portions and a primary connecting median portion), the relative 

 bulk of the latter having decreased in proportion as the brain has 

 increased. 



In all the efforts which have been made to compare or contrast 

 the central nervous system of the Vertebrates with that of the seg- 

 mented Invertebrates (Articulata), the cerebral or supra-oesophageal 

 ganglion of the latter has presented a difficulty which was perceived 

 many years ago by von Baer and has not yet been overcome. 



It is true that it has been urged that the cerebral ganglion of 

 the Annelids is unrepresented in the Vertebrates 2 ). So far, however, 

 as I am aware, there has been no enunciation of any general principle 

 of evolution affecting the praeoral lobe, the appreciation of which 

 would render the non-representation of the Invertebrate cerebral gan- 

 glion in the Vertebrate central nervous system, an intelligible and 

 necessary fact instead of a gratuitous and vague assumption. 



There are many indications that point irresistibly to the conclusion 

 that the prime factor which must be recognised in the evolution of the 

 praeoral lobe, from the relations which it presents in the Invertebrates 

 to those which it holds in the Protochordates and Vertebrates is, its 

 complete emancipation from the central nervous system. 



In the great groups of the Annelids, Molluscs and Arthropods, the 

 praeoral lobe (prostomium, procephalic lobe) is essentially the seat of 

 the cerebral ganglion. The latter, through its representative, the apical 

 plate, is the main and often the sole element of the central nervous 

 system in the Trochophore larva of Annelids and Molluscs. 



1) For reconstructed figure see von Küpffeb, Studien zur vergl. Entw. 

 des Kopfes der Kranioten, München, 1893, Heft I, p. 84. 



2) cf. Beabd, The old mouth and the new. Anat. Anz., Bd. Ill, 

 1888, p. 15—24. 



