ORIGIN AND STRUCTURE OF THE HEAD 41 



formation they closely apply themselves to the stomodaeum, 

 which fact I have proved to my own satisfaction e. g. in 

 pelagic stages of Lanice conchilega, Mytilus edulis (1910) 

 and especially in Littorina obtusata (1913). Transverse 

 sections through this region of the body, showing the 

 stomodaeum flanked by the two statocysts, often remind 

 one of a section through the embryonal medulla oblongata 

 flanked by the two auditory vesicles. 



Now in Annelids the statocysts are always found in the 

 anterior segments of the soma. In the more primitive forms 

 there are several pairs, in Ariciidae e. g. five or six pairs 

 in successive segments, of which the first e. g. in Aricia 

 acustica is found in the ninth segment of the soma. "Chez 

 les Polychetes plus differengies", Fauvel says," les otocystes 

 ne se rencontrent jamais que sur un seul segment qui est 

 le premier segment (buccal ou peristome) pour les Areni- 

 coliens, le deuxieme segment (premier branchifere) pour 

 les Terebelliens et invariablement le deuxieme segment 

 (1" setigere) pour les Sabelliens" (of which 22 species 

 with statocysts are enumerated). The statocysts are always 

 innervated by the ganglia of the segment in which they 

 are found, never by the cerebral ganglia as in Molluscs. 



Auditory vesicles in Craniates. — If we compare with this 

 the state of things in Vertebrates, we find a complete 

 accordance. For if our conclusion is right that the isthmus 

 indicates the place of the old mouth, the first rudiment of 

 the auditory organs here also appears as two vesicles closely 

 behind the mouth, situated on either side of the former 

 stomodaeum, more especially of the anterior part of it, 

 constituting the deuterencephalon. In the transparent 

 larvae of Teleosteans they may be seen to retain for a 

 considerable time the form of a pair of round vesicles, 

 each containing two statoliths. They are, as will be dis- 

 cussed further on, seated in Vertebrates in one of the 

 anterior segments of the soma, probably to be considered 

 as the second, and supplied by a branch of the segmental 

 nerve of that segment, the facialis. This statement on the 

 other hand confirms our conception of the deuterencephalon 

 as the anterior part of the former stomodaeum. 



Praeoral lobe in Craniates, — As the reader will have 

 already observed, another old and important question is 

 brought nearer to its solution by the present considerations, 

 viz: that of the metamerism of the head and the cranium. 



