268 



ZOOLOGY 



SECT. 



Ir 



m, 



APPENDIX TO PLATYHELMINTHES. 



CLASS NEMERTINEA. 



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 (m.) 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 (pr.), 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 Nemer- 

 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, th surface of the extremity is sometimes 



ctir 



Fio. 214. Diagram of the organs of a, 

 Nemertine, from below, a. anus ; Itr. 

 brain ; div. coeca ; long. ne. longitudinal 

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

 ov. ovaries ; pr. proboscis. (After 

 Hubrecht.) 



