CTENOPHORA. 



39 



excr. st. 



hr. 



ph.c. 



a 



freigelegt wird" (p. 652) is a confirmation of the homology between this vessel and the excretory 

 vessel of Ctenophores. Anything corresponding to the other excretory vessel of Ctenophores is not 

 found in the grown Polyclads, (where it would have to be sought for below the brain) but Lang 

 has found an indication of it in the early stages of development. The two accompanying diagrams, 

 copied from Lang show these features clearly. 



The main axis given, the other plans 

 are easily found. The excretory vessels lying 

 always in the sagittal plane, it is accord- 

 ingly the longitudinal axis of the Polyclads 

 which corresponds to the sagittal axis of 

 Ctenophores, the transverse axis of the 

 latter being then also transverse in Poly- 

 clads. From this then naturally follows 

 that the nuchal tentacles of some primitive 

 Polyclads (Plauoceridse) are regarded as 

 homologues of the tentacles of Ctenophores, 

 being placed in the transverse plan. 



The gastrovascular system of Poly- 

 clads and Ctenophores corresponds in seve- 

 ral important points. There is in both a 

 large ectodermal pharynx ("Pharyngeal- 

 tasche" in Polyclads); to the "krausenfor- 

 miger Pharynx" in the more primitive, 

 the "russel" in more spezialized Polyclads, 

 correspond the "Magenwiilste" of Cteno- 

 phores; the fact that in the primitive Polyclads the pharynx is glandular is in accordance herewith. - 

 The pharynx opens in the Polyclads into a "Hauptdarm", of entodertnal origin, corresponding exactly 

 to the infundibulum of Ctenophores. From this proceeds in both groups the main stems of the gastro- 

 vascular system; in the Polyclads there may be many paired stems, but in the more primitive forms 

 they are few in number, though never less than four pairs. In Coeloplana the gastrovascular canals 

 are branching and anastomosing as in Polyclads. In none of the groups is an anal opening found. 

 The histological structure is mainly alike in both; also in Polyclads there is a line of thickened 

 epithelium in the branches of the gastrovascular canals. Finally both agree in the important physio- 

 logical fact, that the food is absorbed in the pharynx. 



The genital organs are of entodermal origin in both ') and both are hermaphrodite. In the 

 Polyclads the testes are situated on the ventral side, the ovaries on the dorsal side of the gastrovas- 

 cular canals, while in Ctenophores they lie respectively in the right and left side of the meridional 

 canals. Important differences are found, however, in regard to the genital organs of the two groups; 

 while the sexual products of Ctenophores are ejected through the gastrovascular system, there are in 



'J In the paper on Gtimia segmentata Lang does not doubt the entodermal origin of the genital organs; in the 

 Monograph he is in doubt about this point. 



Fig. 10. Diagrammatic longitudinal section of a hypothetical ancestral 

 form (a) and of a primitive form of Polyclads (Anonymus) (b); the 

 former corresponds very nearly to Coeloplana. br. places, where the 

 branching gastrovascular canals proceed from the stomach; excr. ex- 

 cretory vessel, in fig. b. the anterior branch of the gastrovascular system; 

 g. ganglion (apical organ); i. m. inner mouth-opening; o. m. outer mouth- 

 opening; ph. pharyngeal folds; ph. c. pharyngeal cavity; s sucking 

 disk; st. stomach (iufundibulum). 9 female genital opening. (Slightly 

 modified from Lang. Monogr. p. 102). 



