232 THE CEPIIALOPHORA. §^ 205, 206. 



upon various points of the body of Sagitta, differ from the locomotive 

 organs of the other Ptcropoda in being eompoKcd wholly of parallel, homo- 

 geneous fibres, which decrease in size from the base to the border of this 

 organ, but which have not the least resemblance to those of muscle.'*" 



Soine Ptcropoda have tentacle-like processes situated in bundles about 

 the mouth, which have a small sucker at their end ; they are, therefore, 

 probably used as organs of attachment.'^' 



§ 205. 



Beside this putaneous muscular system, the cavity of the body contains 

 isolated muscles which serve different uses. With the turbinated Gastero- 

 poda, a large muscle arises from the columella, and, after dividing into 

 many parts, is spread over the sides of the body to be inserted into the foot, 

 and serves as its retractor. Several other muscles of variable size arise 

 also from the columella, and are distributed, some to the tentacles, and 

 others to the pharynx and the penis — serving also as retractors of these 

 organs. With the shell-less Cephalophora, the retractors of these various 

 organs arise from the inner surface of the mantle, or from the foot.'^^ 



CHAPTER III. 



NERVOUS SYSTEM. 



§ 206. 



The central part of the nervous system of the Cephalophora consists of a 

 group of closely approximated ganglia, connected together by several nerv- 

 ous filaments, and which surrounds, like a ring, the base of the pharynx or 

 the oesophagus. This oesophageal ring may be divided into several por- 

 tions ; one situated above, one below, and one each side of the oesophagus. 

 The portion lying above consists usually of two very large contiguous 

 ganglia, which may be called the brain, since they furnish nerves to most of 

 the organs of sense, — that is, the tactile organs, the eyes, and sometimes 

 also the organs of hearing. 



The portion lying below, varies much in its form and size. It consists, 

 sometimes of a group of ganglia blended together, or circularly united by 

 short connecting filaments, and sometimes of a simple transverse cord. 

 The two lateral portions consist always of two cords connecting the upper 

 and lower parts just mentioned. The lower portion, which sends nerves 

 principally to the muscles of the foot and to several viscera, is often asyui- 

 metrical. The peripheric nerves are always given oft' from the ganglia 

 and never from the connecting cords, of the oesophageal ring. 



6 See Krohn, Inc. cit. p. 6. mon), and Eschricht, loc. cit. p. 8, Tab. II. fig. 



7 See Ciivier, .Miin. loc. cit. p. 8, PI. 1. B. fi?;. 12, l:i (Clio). _ 



8 (Pneumodermon) ; D'Orbi^ny, Vuy. dans 1 For tliese isolated muscles, see Cuvier, Mem. 

 I'Auiti-. in^rid.,<>r l.^^ik 1839, p. 49'7, Taf. I. fig. sur la liniacu ct coUma^on, loc. cit. p. 11, PI. IL 

 IX. 1-16 {Sfonsiol/i\ir.cJiafa and Pneumoder- fig. 2, 3. 



