NEMERTINI 



231 



The pilidium resembles the Trochophore of the Annelida (comp. p. 

 266), as well as the Turbellarian larvas. In common with this, it pos- 

 sesses the apical plate, the cords radiating from it, and the ring-nerve 

 which extends under the ciliary apparatus. The apical plate, to be sure, 

 does not give rise here, as in the Annelida, to the oesophageal ganglion, 

 for it is lost with the pilidium. For this reason, it does not seem allow- 

 able to homologize the brain of the Nemerteans directly with that of the 

 Annelida. Apart from this, the lateral nerves of the Nemerteans arise by 

 the growing out of the cerebral ganglion, which is already separated 

 from its connection with the ectoderm, whereas the ventral ganglionic 

 chain of the Annelida appears to take its origin by means of progressive 

 differentiation of the ectoderm. 



The nervous system of the Nemerteans is most closely allied to that of 

 the Platyhelminthes, and particularly to that of the Turbellaria, with 

 which the Nemerteans also present other common features, such as the 

 uniform ciliation of the entire surface of the body, the body parenchyma, 

 and the lateral organs. But the shape of the proboscis appears to us 

 to be of particular value for the comparison of the two groups. The 

 proboscis, situated at the anterior end of the body, apparently having 

 arisen by the metamorphosis of the latter into a tactile organ and being 

 withdrawn into the inside of the body, presents in the two groups a 

 position and structure too much alike not to challenge comparison. 



Other conditions separate the Nemerteans from the Turbellaria. The 

 intestine possesses an anal opening, which is wanting in all Platyhel- 

 minthes. The presence of a differentiated blood-vascular system indi- 

 cates a higher organization of the Nemerteans. The genital organs are 

 constructed quite differently from those of the Platyhelminthes, whereas 

 those of the Turbellaria, Trematoda, and Cestoda show great agreement 

 in structure. In the position of the sexual organs a segmental arrange- 

 ment can be recognized. Whether the indications of segmentation 

 furnished by the presence of the septa which constrict the intestine and 

 the numerous openings of the water system have any higher meaning 

 cannot be said in the present state of our knowledge. What we have 

 hitherto learned in regard to the excretory system (v. Kennel and Oude- 

 MANs) entitles us neither to recognize therein a higher degree of organi- 

 zation nor to place the Nemerteans nearer to the Platyhelminthes, though 

 the presence of two longitudinal vessels might point to the latter. On 

 account of their numerous relations to the Turbellaria (although their 

 organization is much higher than that of the latter), it does not seem 

 justifiable to separate the Nemerteans from the Platyhelminthes altogether 

 and to place them, as has already been done, nearer the segmented 

 worms. The Nemerteans would have to be separated much more 

 sharply from the Turbellaria, if the statements regarding the segmenta- 

 tion of the body and the occurrence of a true body cavity were verified. 



Finally, we cannot leave unmentioned in this place a theory which 

 places the Nemerteans in relation with the Vertebrata. Hubrecht, the 



