NERVOUS SYSTEM. 



183 



ciliograde acalepha possess great activity, are provided with 

 numerous long tentacula exquisitely sensitive and contractile, 

 exhibit distinct internal organs for digestion, circulation, and 

 generation, and are found to be provided with nervous fila- 

 ments and even ganglia around the oesophagus. In the beroe 

 pileus (Fig. 81. A,) the nervous system is disposed around 

 the mouth, at the lower extremity of the body, in form of a 

 double filament (a,) with eight small white ganglia (b,) inter- 

 posed between the eight longitudinal 

 bands of cilia. From each of these gang- 

 lia a small filament proceeds upwards in 

 a longitudinal direction (c,) towards the 

 anal extremity of the body, and others 

 pass out laterally and downwards to 

 the projecting irritable lips surround- 

 ing the mouth. In the medusos a ner- 

 vous band, accompanying the marginal 

 circular continuation of the alimentary 

 cavity, is disposed around the irritable 

 border of the mantle, with minute gan- 

 glia placed near the bases of the marginal 

 tentacula. From each of these ganglia 

 nervous filaments proceed to the near- 

 est tentacula, along the bases of which they can be traced. 

 Eight optic ganglia are perceived at the bases of the eight 

 ocular peduncles, from which nerves proceed to the small 

 red coloured marginal eyes, which they are observed to en- 

 ter. Other ganglia are also perceived, near the ovaries, 

 around the entrance to the stomach, from which nervous 

 filaments, are seen passing downwards to the central groups 

 of tentacula. From each optic ganglion two nerves proceed, 

 which unite by a kind of decussation before they enter the 

 eye ; the eye is provided with a distinct crystalline lens, and 

 the pigment is red-coloured, like that of most of the simplest 

 forms of this organ. From the active movements of most 

 acalepha in swimming through the sea, from their gregareous 

 habits, the exquisite sensibility and contractility of their ten- 

 tacula, the distinct development of their muscular fibres, and 

 their perception of light, it is not likely that many are desti- 

 tute of some form of this system, so general in its occurrence 

 and so influential in the economy. 



