228 COMPARATIVE ANATOMY CHAP. 



in the epithelium all over the body. The motor nerves of the supra- 

 oesophageal ganglion form a ganglion each on the sides of the fore-gut 

 in the cephalic segment (lateral cephalic ganglia), Avith small accessory 

 ganglia. The musculature of the head and the fore-gut are supplied with 

 nerves by these ganglia. This mesodermal part of the nervous system 

 of Sagitta recalls the oesophageal nervous system of other worms. We 

 do not yet know how the trunk and caudal musculature are supplied 

 with nerves. 



VIII. Sensory Organs. 



All the different kinds of sensory organs are found in the worms- 

 organs of touch, sight, hearing, smell, and taste. Besides these, in a 

 few divisions we meet Avith sensory organs which cannot at present be 

 classed in any of the aboA r e categories the lateral organs of the 

 Chcetopoda, the lateral eyes of Polyophthalmus, and the segmental organs 

 of the Hirudinea. It must not be forgotten that the functions 

 of the sensory organs of the Avorms are as little experimentally 

 established as are those of most other invertebrate animals ; and that 

 it is almost entirely their position and structure Avhich lead us to 

 consider them auditory, olfactory, etc. The function of the organs of 

 touch indeed is more surely established. That the worms in some 

 Avay or other see Avith the sensory organs which haA^e been indicated 

 as eyes may also be considered certain, though we do not knoAv what 

 and how they see. 



The sensory organs are most numerous and best developed in 

 animals leading a free aquatic life (Polychceta, Errantia, Chcetognatka), and 

 among these again the good sAvimmers take the first place. The 

 worms Avhich are attached at the bottom of the water and those 

 which lurk in holes are not quite so fully provided. In worms living 

 in mud and sand or in earth the sensory organs are much reduced, 

 and this is the case in the highest degree in parasitic and attached 

 animals. In the latter, hoAvever, the strongly developed organs of 

 touch form an exception. Where the sensory organs are reduced in 

 adult, stationary, or parasitic animals AVC often meet Avith them Avell 

 developed in their young stages, Avhen they as larvae move about freely. 

 In order of frequency Ave have the organs of touch, Avhich are universally 

 distributed, then the eyes, then the olfactory organs and organs of 

 taste. Organs of hearing have been certainly observed only in a feAv 

 cases (Arenicolidce, Serpulacea, Terebelloidea). 



A. Organs of Touch. 



EveryAvhere, except in the Avorms provided Avith a thick shell, the 

 entire integument is the seat of a highly developed sense of feeling 

 or touch. This sense is served in a special manner by epithelial sen- 

 sory cells, Avhich carry at their free end sensory hairs or setae, and at 

 their basal ends are continued as nerve fibres, Avhich are themselves 



