54 



CTENOPHORA. 



and these two points are in good accordance with the greater specialization of the pharyngeal appa- 

 ratus in Polyclads. It thus appears that the homologizing of the gastrovascular system in Polyclads 

 and Ctenophores, instead of being "wenig befriedigend", must be declared to be satisfactory, even to 

 an uncommon degree. - - It should also be pointed out that the fact of the oesophagus of Polyclads 

 being a longitudinal slit is another important support for the homology of the longitudinal axis of 

 Polyclads with the sagittal axis of Ctenophores. 



The homologies of the nervous system are somewhat more questionable, the main difficulty 

 lying in the rather disputable character of the nervous system of the Ctenophores. But also here the 

 more recent researches have brought to light facts which are in favour of the theory of the derivation 

 of the Polyclads from the Ctenophores. Lang has maintained as "ein unabweisbares physiologisches 

 Postulat" that the adaptation to the creeping mode of life in Coeloplana must have modified to a con- 

 siderable degree both its musculature and its nervous system. The researches of Abbott have 

 shown that this is really the case; while he does not enter on a more detailed description of its 

 muscular system, he gives the important information that four distinct ganglia have developed round 

 the apical organ (Op. cit. p. 61). Also in Ctenoplana ganglia appear to have developed (cf. Korotneff. 

 Op. cit p. 248). On the other hand the sessile Tjalfiella does not show any indication of ganglia. 

 These facts would seem to show that the change of habits from pelagic to creeping has been of 

 primary importance for the development of the nervous system. ( It would be of unusual interest 

 to study Mertensia ovum in regard to its nervous system; on account of its strongly developed muscular 

 system it should be expected that also the nervous system has been considerably more developed than 

 in other pelagic Ctenophores ). Adding to these facts that Samassa (Zur Histologie d. Ctenoph. 

 p. 229 231) upon histological and physiological grounds conies to the result, that the development of 

 the Polyclad nervous system from the apical organ and ciliated ridges of Ctenophores is highly prob- 

 able, it must certainly be agreed that this point of the theory has been strengthened by the recent 

 researches. 



The otolith itself, being doubtless an adaptation to the pelagic life, should be expected to have 

 become rudimentary in the creeping forms. This is also decidedly the case in Tjalfiella, whereas it 

 is still typically developed in (Coeloplana and Ctenoplana, deviating from that of pelagic Ctenophores 

 only in minor points. The polar fields, which have totally disappeared in the grown Tjalfiella, have 

 been somewhat specially developed in Ctenoplana ("sensory tentacles" of Willey); their condition in 

 Coeloplana is unknown, being not mentioned by Abbott. (It may be suggested that the two vertical 

 tracks from the otolith in Fig. 6, Taf. 8, of Abbott's memoir may represent the polar fields). In the 

 Polyclads both otolith and polar fields have disappeared completely. The presence of otoliths in the 

 Acoela I would regard as a new formation, being of quite another structure than in the Ctenophores. 



No homologue of the eyes of Polyclads has hitherto been found in Ctenophores. It is, how- 

 ever, quite possible that the whitish-yellow spots occurring along the periphery of Coeloplana repre- 

 sent the first rudiments of organs sensitive to light, as suggested by Abbott. Also in Ctenoplana 

 a series of (red) spots occur along the margin of the body, which even more than those of Coeloplana 

 are suggestive of eyes. The fact that in the more primitive Polyclads eyes occur along the margin 

 all round the body is in full accordance herewith, so that it seems highly probable that the origin of 



