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possess very little pigmentation, and may in addition be very 
scattered. 
For this reason, one must feel a little doubtful towards the 
assertion that Stylochus nebulosus Girard lacks marginal eyes when 
mature, while, on the other hånd, the young specimens possess 
such. Meixner uses this argument in his treatise, when he considers 
the arrangement of the eyes in the genus Stylochus a systematic 
characteristic of not very great importance (1907, p. 115), basing 
his statement on Verrill’s contention. 
The individual fluctuation in the number of the eyes hampers 
to a certain extent their use in the systematic classification of 
species. The faet that the number of the eyes inereases with the 
growth of the animal is well-known. This is especially noticeable 
in species with numerous eyes. Moreover, as a difference in number 
is found on the left and the right sides of the same specimen, one 
is at first rather indisposed to the practical use of this charac¬ 
teristic. A careful examination discloses the faet, however, that 
the fluctuation concerns more the number of the ocelli than their 
location and grouping. Moreover, examining a larger amount of 
specimens, one finds that, on the whole, the arrangement of the 
ocelli in both halves of the body corresponds fairly well. This applies 
not only to the size, but also, which is of more interest, to the 
direction of the openings of the eye-cups. The significance of the 
latter condition for the localization of the animal’s movements can 
hardly be misconceived. In this connection I might mention that 
some of the cerebral eyes are located at a lower level in the body 
and approach the ventral muscle wall. The dissimilarity in the 
number of the eyes of the two halves of the body, when this is 
the case, may, at least partly, be due to the faet that one large 
eye on the one side corresponds to the two smaller ones on the 
other; i. e. that a division of the corresponding eye has oceurred. 
This faet is more easily established in Polyclads where the number 
of eyes is small. Certainly other irregularities may oceur than those 
which can be interpreted as caused by unequal contraction of the 
tissue. The more the number of ocelli inereases, the more they 
tend to show a deviation from the symmetrical distribution. I 
suppose that this can be attributed to the circumstance that the 
groups of eyes rather than the individual ones are concerned as 
regards the movements of the animal. 
