s6 



CTENOPHORA. 



main axis has taken place, is evident from the reduction of the infundibular canal in these forms, the 

 excretory canals proceeding directly from the infundibulum. The dorsal side of Coeloplana thus 

 correponds to the whole of the body surface of a Pleurobrachia. Another question here arises, 

 viz. whether in the Polyclads there is anything corresponding to the transverse 

 furrow of the creeping Ctenophores. It seems that such is really the case. Among the processes 

 of the pelagic Polyclad-larva there is a rather broad, prseoral, ventral process, which would appear to 

 correspond to the one lobe; the paired posterior processes accordingly would correspond to the other 

 lobe, having only become secondarily divided. Whether the unpaired dorsal process morphologically 

 also belongs to this lobe seems more questionable; it may perhaps represent a new formation. It is 

 worth recalling in this connection that Lang (Monograph p. 380) seems somewhat in doubt, whether 

 the ciliated band connecting the paired processes is really in connection with that of the anterior 

 ventral process. That the processes are in the transverse plane, is in accordance with the suggestion 

 of their being homologoiis to the transverse lobes of Tjalfielta and Ctenoplana. That they are quite 

 resorbed during the metamophosis of the Polyclad would not seem to make such an homology impos- 

 sible. A consequence of this suggested homology would be, that only the middle part of the ventral 

 side of Polyclads corresponds to the whole of the ventral side of Coeloplana and Ctenoplana, the 

 Polyclads thus representing a further stage of development in the transformation from pelagic to 

 creeping organisms. 



This question about the homology of the ventral side of the Polyclads has been answered in 

 another way by those of the previous authors, who have paid attention to it. While Lang appears 

 to think the flat shape of the Polyclads the result of a simple shortening of the main axis, the edge 

 of the Planarian body thus corresponding to the equatorial zone of the Ctenophoran body, Hatschek 

 (Lehrbuch d. Zoologie, p. 319) points out that Coeloplana and Ctenoplana "wahrscheinlich nicht mit 

 der aboralen Flache kriechen, sondern mit dem ausgebreiteten Schlunde, wie dies auch andere Cteno- 

 phoren gelegentlich thun". Likewise K. C. Schneider") regards the flat oral side of Coeloplana and 

 Ctenoplana as homologous with the lower jDart of the pharynx of other Ctenophores, the well known 

 Lampetia pancerina Chun, which even uses the inverted pharynx for creeping, being taken as the 

 proof of this homology. — The morphology of Tjalfiella and Ctenoplana decidedly gives no support 

 for this theory. It is the transverse furrow which becomes the flat underside, in the middle of 

 which is the opening of the pharynx. In Lampetia there is no trace of a transverse furrow, the ever- 

 sion of the sagi tally compressed pharynx being morphologically quite a different thing, though it 

 may physiologically serve the same purpose, viz. to form a flat surface adapted to perform creeping 

 movements. 



The suggested homology of the larval processes of the Polyclads with the transverse lobes of 

 Tjalfiella and Ctenoplana^) would also seem to throw light on the homologies of the ciliated baud 

 encircling the margin of the processes of the Polyclad-larvse. In Tjalfiella and Ctenoplana the whole 



") Lehrbuch d. Histologic, p. 184. — Systematische Stellung von Hydroctena salenskii. Zool. Auz. XXVII. 1904. p. 570. 



2) As mentioned above (p. 45) Kemna suggested that the larval processes of the Polyclads were homologous to the 

 lobes and auricles of Lobatae. This was evidently a happy thought, representiug an important step in the right direction. 

 That the auricles have nothing to do with the larval processes is clear enough now. But at that time it might not seem so 

 improbable — though it is, of course, always dangerous to seek for the origin of some Uttle speciaUzed structure in a very 

 differentiated organ of some highly specialized forms, as the Lobatse undoubtedly are. 



