217 
That the eyes also of Arthropods, which in their situation 
and their relation to the cerebral ganglia so closely agree 
with those of worms, are to be derived from the same 
starting point cannot be doubted. 
Absence of paired eyes in Acrania. — Now in Amphioxus we 
should expect to find a pair of eyes on the surface of the 
prostomium, in front of the neuropore, corresponding to 
those of Annelids. Neither eyes, however, nor cerebral 
ganglia are to be found here; evidently we must assume 
that they have been lost as a result of the change of 
function of the snout, which in Amphioxus serves for 
burrowing into the sand while in Ascidians, according to 
WILLEY (1894), it is the same organ that serves for attachment. 
Optic pits in Craniates. — If in Craniates, however, the 
praechordal region of the body may be compared to the 
DE praeoral lobe in Annelids, 
ontogenetic stages a dispo- 
sition which strongly re- 
minds us of what is found 
in Annelids. In the devel- 
opment of Craniates the 
Ea first indication of the retinal 
area is often noticed already 
on the still open and flat 
with 10 mesoderm segments. _praechordal cerebral plate 
s a pair of shallow de- 
2, p. 156, after a modell of KEIBEL. pressions of the thickened 
ectoderm. As well in Elasmobranchs and Amphibians as 
also in Mammals the occurrence of these optic pits has 
been known for a long time. The cerebral plate with the 
eye-pits, situated in front of the provisory neuropore, 
reminds us strongly of the apical plate of the trochophora 
with the rudiments of the eyes situated in front of the 
mouth, especially in such cases where the medullary plate 
has closed while the cerebral plate is still open, and where, 
accordingly, the anterior opening of the former represents 
a kind of provisory neuropore corresponding to that of 
Amphioxus and, if our conclusions until now are right, to 
the mouth of Annelids. An example of such a case is given 
by fig. 8, after KEIBEL. Especially interesting in this respect 
are the observations of EYCLESHEIMER (1893, 1895) on 
Fig. 8. Rostrolateral view of the 
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