241 
as derived from the primary entoderm. The contrast between 
the “primary mesoderm or embryonal mesenchyme” and 
the “secondary or coelomatic mesoderm” was first empha- 
sized by MEYER (1890) who compared the former to the 
mesenchyme of plathelminthes and derived the coelom:c 
pouches from the genital follicles of the latter (gonocoel- 
theory). Thesefollicles often even already exhibit a tendency 
to regular metameric arrangement. According to this view 
we may consider the ectodermal mesenchyme of the trocho- 
phora as a last remnant of that of mesenchymatous worms, of 
their larva, the protrochula (HATSCHEK) — MüLLER's larva of 
Polyclads, pilidium and DESOR's larva of Nemerteans—and 
of the Ctenophores, in the same way as the protonephridial 
head-kidney found in several trochophoras reminds us of 
the richly branched excretory system of ancestors like the 
mesenchymatous worms or the Rotatoríia. 
Praeoral lobe in Amphioxus. — The mouth belongs to the 
first trunk segment, the peristomium. It is situated just 
behind the limit of the prostomium and the first segment. 
w, i compare this with what we find in a young 
embryonic stage of Amphioxus, as represented in fig. 5 and 
‚there is a remark- 
prost. | soma able agreement to 
EE TETE be noticed. In front 
of the first pair of 
coelomic pouches 
we find here also a 
part of the body in 
which originally no 
coelomic mesoderm, 
and even no meso- 
derm at all, ís pre- 
: \ red sent, and which we 
Fig. 5. Optic rs ANR rn of a can compare to the 
oung s 
ade ol APN) _pracoral_ lobe 
is here indicated by the first pair of coelomic pouches. 
The agreement so far hardly could be more complete. 
