ORIGIN AN D STRUCTURE OF THE HEAD 45 



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 

 families ou Ton ne connait pas sa presence, le plus 

 souvent pour ne pas I'avoir cherche, contiennent les for- 

 mes tres degradees et surement pas primitives. On pent 

 done dire que Forgone nucal est un organe typique da 

 lobe cephalique des Polychetes ce qui veut dire qu'il est 

 herit^ de la souche et non une nouvelle acquisition, ou 

 encore que tous les Polychetes I'ont ou ont du I'avoir 

 a un certain stade de leur developpement embryonnaire ou 

 phylogenetique." 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 



