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equal size, of which the brain of these forms is composed, 
may be justly called “ganglia of the ciliated organs”, while 
the anterior pair innervates the eyes and the surface of the 
praeoral lobe. KLEINENBERG (1886, p. 71) has shown that 
ontogenetically the posterior pair of ganglia arise in 
closest connection with the olfactory pits, to which they 
evidently owe their existence. This is confirmed by KEPNER 
and CASH (1915 p. 245) who, after having studied the 
development of the ciliated pits in a flatworm, make the 
following remark: “In conclusion, it is interesting to observe 
the striking parallelism presented by this organ in its 
function and mode of origin with the olfactory organ of a 
vertebrate so far as its function (i.e. its function in the 
fish) and its mode of origin is concerned.” 
RACOVITZA (1896), after having enumerated a great num- 
ber of species provided with ciliated organs, says: “Les 
familles où l'on ne connaît pas sa présence, le plus 
souvent pour ne pas l'avoir cherché, contiennent les for- 
mes très dégradées et sûrement pas primitives. On peut 
donc dire que Porgane nucal est un organe typique dut 
lobe céphalique des Polychètes ce qui veut dire qu'il est 
hérité de la souche et non une nouvelle acquisition, ou 
encore que tous les Polychètes l'ont ou ont dû [avoir 
à un certain stade de leur développement embryonnaire ou 
phylogénétique”’ RACOVITZA subdivides the ciliated organs 
ot Annelids into five categories; the pits can be more or 
less deep and may be capable of eversion or not. Also in 
Planarians, Nemerteans and Molluscs we find similar ciliated 
pits in corresponding places, of which those of Cephalo- 
pods were already mentionned above. They are situated 
behind the eyes. In Nautilus and in Opisthobranchs the 
rhinophores have the form of prominent appendages, no 
doubt to be derived from the devaginable olfactory organs 
of certain Annelids. 
As indicated above, the ciliated organs are usually found 
near or at the posterior border of the prostomium on the 
dorsal side (hence their name “nuchal organs”). Such 
forms as the trochophora of Polygordius (cf. WOLTERECK, 
1902) show clearly that there can be no doubt that they 
belong to the organs originating from the surface of the 
prostomium, though they often lie close to its border. The 
peripheral cells of the episphere, round the little apical 
plate sensu stricto, are in this form extremely flattened and 
