268 



ZOOLOGY 



SECT. 



ong.ne 



APPENDIX TO PLATYHELMINTHES. 



CLASS NEMERTIXEA. 



General Features. The Nemerteans are non-parasitic, unseg- 

 mented worms, most of which are marine, only a few forms living 

 on land or in fresh-water. They are commonly looked upon as nearly 

 related to the Turbellaria, and were formerly included in that 

 class ; but they are in some respects higher in organisation than 



the Turbellaria, and they ex- 

 hibit certain special features 

 distinguishing them from the 

 rest of the lower Worms. 



The body (Fig. 214) is nar- 

 row and elongated, cylindrical 

 or depressed, unsegmented, and 

 devoid of appendages. In length 

 it varies from a few millimetres 

 to as much as ten, metres. The 

 entire surface is covered with 

 vibratile cilia. Frequently the 

 integument is vividly coloured. 

 The cells of the epidermis secrete 

 a mucous matter, which may 

 serve as a sheath or tube for the 

 animal. The mouth (//;.) is at or 

 near the anterior extremity on 

 the ventral aspect. Close to it 

 above there is an .opening, 

 through which can be protruded 

 a very long muscular organ, the 

 proboscis (?'.), the possession of 

 which is one of the most char- 

 acteristic features of this class of 

 Worms. The proboscis is hollow ; 

 when it is extended to its ut- 

 most, a part still remains which 

 is not capable of being everted, 

 and at the junction between the 

 eversible and non-eversible parts, 

 i.e. at the extremity of the 

 organ when it is fully protruded, 

 there is in many of the Xemer- 

 teans a pointed or serrated stylet (Figs. 215. 217, and 218), 

 which probably permits of the proboscis being used as a weapon : 

 when a stylet is absent, the surface of the extremity is sometimes 



Fin. 214. Diagram of the organs of a 

 Xeniertine, from below, a. anus ; &/. 

 brain; die. oceca ; fonri.ne. longitudinal 

 nerve-cords ; ;. mouth ; n. nephridia ; 

 ov. ovaries ; pr. proboscis. (After 

 Hubrecht.) 



