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formation they closely apply themselves to the stomodaeum, 
which fact 1 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 Ariciïdae 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 Polychêtes plus différengiés’”’, FAUVEL says,“ les otocystes 
ne se rencontrent jamais que sur un seul segment qui est 
le premier segment (buccal ou péristome) pour les Aréni- 
coliens, le deuxième segment (premier branchifêre) pour 
les Térébelliens et invariablement le deuxième segment 
(ler sétigère) 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. 
