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head and trunk, emphasizing the separate origin of the 
nervous system (cerebral and ventral ganglia) in the two. 
HATSCHEK (1878, p. 69), as a result of his researches 
on Polygordius, had used the word head in a somewhat 
different sense, comprising not only the umbrella but also 
the subumbrella of which the segmented soma is considered 
by him to be only a kind of outgrowth or appendix. So to 
this “head segment’, corresponding to prostomium + peri- 
stomium (first segment of the soma), not only the mouth, 
which lies immediately beneath the prototroch, but also the 
protonephridia and the statocysts were considered to belong 
(cf. anon). KLEINENBERG, however, emphasized that the 
development of the larva of Polygordius, where a great 
deal of the umbrella and the subumbreila, together with 
the prototroch, is cast off during metamorphosis, exhibits 
somewhat peculiar features which may easily lead to misinter- 
pretation. In general it is quite evident in Annelidan larvae 
that the first segment, the peristomium, containing the 
first pair of ventral ganglia (the infra-oesophageal ganglia), 
lies immediately behind the prototroch. The prototroch 
accordingly must be considered as the limit of the non- 
segmented head or prgstomium and the segmented trunk, 
and the mouth and the statocysts as belonging to the 
latter. This view has found general acceptance among later 
investigators of which Il will mention only SALENSKY (1887 
p. 632), MEYER (1890 p. 296), RACOVITZA (1896), GOODRICH 
(1897) and EISIG (1899, p. 226). 
Mesenchyme and coelomesoblast. — The prostomium (a term 
introduced by HUXLEY) contains originally the cerebral 
ganglia which originate from its wall, though in Oligochaeta 
they secondarily may wander backwards into the anterior 
trunk segments. The latter contain each a pair of ventral 
ganglia and a pair of mesoblastic somites which surround 
a portion of the alimentary canal. The cavity of the prosto- 
mium, however, belongs to HATSCHEK’s primary body-cavity, 
the blastocoele; it is, as GOODRICH (1897) remarks, “primiti- 
vely of the nature of a blood-space, most clearly seen in 
trochosphere larvae, where it is much enlarged.” The 
scattered mesenchymatous cells originally contained in it 
have an ectodermal and probably a radial origin, as shown 
by the cell-lineage investigations, whereas the trunk- or 
coelomatic mesoderm is produced in a bilaterally sym- 
metrical way by the two teloblasts which must be considered 
