288 ZOOLOGY sect. 



important differences between a Cestode and a Trematode, in 

 addition to the absence of an enteric cavity in the former and 

 its presence in the latter, is the occurrence in the Cestodes of 

 strobilation. Ligula in a certain sense forms a connecting link in 

 this respect between the Trematode and the ordinary Cestode, the 

 body being elongated and the reproductive organs repeated as in 

 the normal Tape- Worm, but there being no corresponding division 

 of the body into a string of definitely separated proglottides. 



Of importance in connection with the subject of the relationship 

 of Trematodes and Cestodes is the question whether the scolex of 

 the latter is at the end corresponding to the anterior end of the 

 former, or whether it is the free end of the strobilia that is in 

 reality anterior. In favour of the latter conclusion is the fact 

 that the hooks of the hexacanth larva, developed at its anterior 

 end, are found in the cysticercoid to lie in the tail region, i.e., the 

 region most remote from that which develops the scolex, and thus 

 at the end which should represent the free extremity of the 

 strobila. On the other hand, the specialisation of the nervous 

 system to form quite definite and comparatively elaborate nerve- 

 centres (brain) in the scolex of some Cestodes {e.g., Moniezia) 

 tells in favour of the view that the scolex is anterior and 

 corresponds to a head. 



APPENDIX TO PLATY HELMINTH ES. 



Class Nemertjnea. 



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 in some respects they are higher in organisation than 

 the Turbellaria, and they exhibit certain special features distin- 

 guishing them from the rest of the lower Worms. 



The body (Fig. 228) is nearly always narrow and elongated, 

 cylindrical or depressed, unsegmented and devoid of appendages. 

 In length it varies from a few millimetres to as much as ten 

 metres. In some cases there is a short narrower posterior region 

 or " tail " ; a head is rarely marked off from the body proper. 

 The entire surface is covered with vibratile cilia, and frequently the 

 integument is vividly coloured. Gland-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. Near it in front (rarely united with it) 

 there is an opening through which can be protruded a very long 

 muscular organ, the proboscis (pr.), the possession of which is one 



