232 THE CEPHALOPHORA. $$ 205, 206. 
upon various points of the body of Sagitta, differ from the locomotive 
organs of the other Pteropoda in being composed 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. 
Some Pteropoda 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 cutaneous 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 cesophagus. This cesophageal ring may be divided into several por- 
tions; one situated above, one below, and one each side of the cesophagus. 
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 asym- 
metrical. The peripheric nerves are always given off from the ganglia 
and never from the connecting cords, of the esophageal ring. 
6 See Krohn, lov cit. p. 6. mon), and Eschricht, loc. cit. p.8, Tab. II. fig. 
7 See Cuvier, Mem. loc. cit. p. 8, Pl. 1. B. fig. 12, 13 (Clio). 
8 (Pneumodermon) ; D'Orbigny, Voy. dans 1 For these isolated muscles, see Cuvier, Mém. 
PAmeér. mérid., or Isis, 1839, p. 497, Taf. I. fig. sur la limace et colimagon, loc. cit. p. 11, Pl. IL 
IX. 1-15 (Spongiobr h and Pne der- fig.2,8. ~  - 
