m 



PLATHELMINTHES 177 



According to the recent investigations of Metschnikoff (No. 16) on 

 Ctenophores, the embryonic development of this group offers some 

 resemblances to that of the Polyclads. The ectoderm cells are constricted 

 off from the four blastomeres originally present, and grow over them from" 

 above. As in the Polyclads, four mesodermal groups are present here, 



d likewise take their origin, although in a somewhat different way, 



m the large blastomeres. The subsequent deportment of the meso- 



rmal tissue is similar in the two groups in so far as it fills the entire 

 space between ectoderm and entoderm. Since among the lower forms 

 the Ctenophora are the only ones which present mesodermal tissue of this 

 kind, there is in that fact an argument for placing the Turbellaria in 

 relation with them. 



The oiten-attempted comparison of the systems of organs in Ctenophora 

 and Turbellaria, especially that of the gastrovascular apparatus, is, how- 

 ever, less satisfactory. On the contrary, Lang's suggestion concerning the 

 position of the cilia and the manner of their, motion in the Turbellarian 

 larvae appears to us to be of some importance. The cilia are arranged in 

 regular transverse rows on the ciliated band, and all the cilia of a trans- 

 verse row move at the same time in a manner which quite recalls the 

 strokes of the swimming plates of the Ctenophora. If the cilia of a row 

 were to fuse with one another, says Lang, then the structure arising in 

 this way could not be distinguished from such a swimming plate. But 

 how far Lang's attempted homology of the eight ciliated lobes with the 

 ribs of the Ctenophora has claim to validity is very doubtful. 



Possibly also the brain of the Turbellaria can be referred to the apical 

 plate of the Ctenophora. It has been determined embryologically by 

 Lang that the originally aboral pole of the embryo becomes shifted 

 toward the anterior end of the body. The brain then arises in that 

 region. If the displacement were not to take place, then the brain would 

 arise at the aboral pole, and would consequently have the same position 

 as the apical plate of the Ctenophora. Even the otocysts of the Turbel- 

 laria, which in certain forms {Monotidce, Otometsostoma, according to v. 

 Gkaff) lie close to the brain, are perhaps to be looked upon as the re- 

 mains of the otocysts of the Ctenophora. 



Nevertheless it is to be emphasized that the Turbellaria and Cteno- 

 phora, even if they proceeded from a common root, have become so much 

 altered that the comparison can be of only a general nature. We have 

 already mentioned (p. 157) that we do not ascribe to the intermediate 

 forms Coeloplaua and Ctenoplana (Nos. 10 and 11), proclaimed as uniting 

 links between the Ctenophora and Turbellaria, any such meaning. 

 Nevertheless such forms are, in our opinion, valuable in showing how the 

 transition from free -swimming, radial animals into creeping, bilateral 

 forms could have been accomplished. 



K.H. E. 



