IV VERMES— SENSORY ORGANS 229 



generally processes of peripheral ganglion cells. A plexus of nerve 

 fibres and ganglion cells lying immediately under the body epithelium 

 can now be proved with certainty to exist in various worms, especially 

 in the Chcetopoda and Chwtognatha. In parasites with a thick outer 

 cuticle papillae, rod- or seta-like processes of this cuticle, which are 

 principally developed at the anterior end and near the genital apertures, 

 play the part of organs of touch. As a rule the tactile cells are most 

 numerously present in that part of the integument of the worm where 

 the body has most points of contact with its immediate surroundings. 

 Such points are, above all, the anterior end of the body, the neighbour- 

 hood of the mouth, and the various ajjpendages. There are some 

 such appendages which, on account of their position and their speci- 

 ally rich provision of tactile cells, may be regarded as specific 

 organs of touch. We may mention in illustration the feelers on 

 the heads of the Chcetopoda, especially of the Pohjchceta, the cirri of 

 the parapodia, the prostomium of the EcMuridcc, the tentacles of 

 the Prosopalgia (cirri on the oral arms of the Brachiopoda), and the- 

 wheel organ of the Rotatoria. It is obvious that such organs of 

 touch in worms inhabiting tubes or shells can only attain development 

 at the anterior end of the body, which carries on the relations with 

 the outer world ; and it is equally intelligible that in such worms 

 these same organs should carry on other relations to the outer world 

 as well (prehension of food, respiration). Hence the strong develop- 

 ment of the tentacular apparatus in the tubicolous and shelled worms 

 (tubicolous Annelids, most Prosopygia, and the tubicolous Rotatoria). 



The sense of touch is very strongly developed in the Hirudinea. 

 The tactile cells, each of which is provided with a fine hair, form 

 groups (tactile cones), which are arranged in 18 longitudinal rows. 

 They are developed on the warts or papilla, when such occur (Fig. 156). 



B. Eyes. 



Their oeeurrenee, number, and arrangrement. — In many genera 

 and species of Nemertina the eyes are wanting. In others small 

 eyes occur in varying numbers (2, 4, or many) at the anterior end 

 of the body. In the Nematoda the absence of eyes is the rule, the 

 presence of 2 simple eye-spots at the anterior end of the body (in 

 some of the free-living forms) the exception. The Acanthocepihala 

 are without eyes. In the Annulata the presence of eyes is the 

 rule, their absence the exception. The Hirudinea possess 1 to 5 pairs of 

 eyes in the anterior rings of the body. Among the Oligocluda, which 

 mostly live in mud or earth, only the Naidomorpha possess eyes — one 

 pair in the head segment. In the Archiannelida which have a similar mode 

 of life, or are, like Histriodrilus, parasitic, the eyes are either wanting 

 or reduced in the adult animal. Most of the Polychceia are provided 

 with eyes, which lie, with very few exceptions, in the head segment. 

 Most of the Errantia have 1 or 2 pairs of well-developed eyes, which 



