CTENOPHORA. 



39 



excr. st. 



a 



freigelegt wird" (p. 652) is a confirmation of the horaolog\- between tliis vessel and tlie excretory 

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

 found in the grown Pohxlads, (where it would have to be sought for below the Inainl 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 trans\'erse in Pol\- 

 clads. From this then naturally follows 

 tliat the nuchal tentacles of some primitive 

 Polyclads (Planoceridte) 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- 



Fig. 10. Diagrammatic longitiulinal section of a hypothetical ancestral 



ral important points. There is in both a form (a) and of a primitive form of Polyclads (.■hionymus) (b); the 



, . 1 , , ,,,^, , former corresponds very nearly to Codoplana. br. places, where the 



large ectodermal pharynx ("Pharvng-eal- , , ■ \ 1 T At n ^ %, 



'^ 1 . 1 fi branching gastrovascular canals proceed from the stomach; excr. ex- 



tasche" in Poh'clads); to the "krausenfor- cretory vessel, in fig. b. the anterior branch of the gastrovascular .system; 



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



miger Pharynx" in the more primitive, opening; ph. phar>-ngeal folds; ph. c. pharyngeal cavity; s sucking 



the "riissel" in more spezialized Polvclads '^'*'^- **■■ stomach (infundibulum). 9 female genital opening. (Shghtly 



modified from Lang. Monogr. p. 102) 

 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 entodermal origin, corresponding exactly 

 to the infundibulum of Ctenophores. P'rom this proceeds in both groups the main stems of the gastro- 

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

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

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

 The histological structure is mainlv 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 thev lie respectivel\- 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 m 



M In the paper on Gunda segmentata Lang does not doubt the entoderinal origin of the genital organs; in the 

 Monograph he is in doulit about this point. 



