MOVEMENTS OF POLYPES. 249 



tion of sensation. There would appear to be little difference in the character 

 of this movement, between the simple Hydra and the most perfect Vertebrated 

 animal. In the latter, however, another set of muscles are superadded to 

 these, for the purpose of preparing the aliment by mastication for the opera- 

 tion of the stomach, and of bringing it within reach of the pharyngeal con- 

 striction. But, it has been urged, the inactivity of the tentacula when the 

 Hydra is gorged with food, proves that they are excited to action by the will 

 of the animal. This inference, however, may be easily disproved. The 

 muscles of deglutition in Man are not called into action with nearly the same 

 readiness and energy, when the stomach is distended, as when it is empty; a 

 fact of which any one may convince himself, by observing the relative facility 

 of swallowing, at the commencement and the termination of a full meal. No 

 one will assert that this variation is an effect of the will; indeed, it is often 

 opposed to it; being one of those beautiful adaptations, by which the welfare 

 of the economy is provided for, but which the indulgence of the sensual appe- 

 tites opposes. Most of the movements of this animal, and of others of the 

 class, appear to be equally the result of external stimuli, with that already 

 described; and it is only in a few instances, principally those of absolute 

 locomotion or change of place, that any evidence of voluntary action can be 

 discerned. It may be occasionally remarked, however, that one or more of 

 the tentacula are retracted or extended, without the slightest appreciable 

 change in any of those external circumstances, which seem ordinarily to 

 affect the motions of the animal ; and this action we can scarcely regard as 

 otherwise than voluntary. 



314. Thus in the Nervous System of Radiated Animals, we have an in- 

 stance of that community of function, which is so remarkable in the organism 

 of the lower tribes, when contrasted with the separation, which is perceptible 

 in those at the opposite extremity of the scale. The visceral nerves of the 

 Asterias are not isolated at their central terminations from those which are 

 connected with the sensorial and locomotive functions : nor are the nerves 

 which minister to the instinctive actions separable from those which convey 

 the influence of the will. Every segment of the body appears equal in its 

 character and endowments to the remainder; each has a ganglion appropriated 

 to it; and, as the ganglia, like the segments, are all alike, neither of them can 

 be regarded as having any presiding character. 



315. From the Radiated we now pass to the MOLLUSCOUS classes; the 

 general character of which, as a natural group, is the remarkable predomi- 

 nance of the Nutritive system over that of Animal life. There is not in the 

 Mollusca, as in the Radiata, any repetition of parts around a common centre; 

 and we do not therefore meet in them with a number of ganglia, nearly or 

 altogether alike in endowments. In some of the higher species, there is a 

 conformity between the two sides of the body, or a lateral symmetry; which 

 involves a subdivision of some of the ganglia, that are single in the inferior 

 tribes, into two masses, which always remain in connexion with each other. 

 With this exception, it may be observed, that all the principal ganglia, to the 

 number of four or five, which we meet with in the higher Mollusca, appear 

 to have distinct functions; as may be determined by tracing the distribution 

 of their nerves. Thus we find a pair of cephalic ganglia, situated above the 

 esophagus, connected with the organs of special sensation, and sending motor 

 nerves (as we shall see reason to believe) to all parts of the body. This is 

 obviously analogous to the brain of Vertebrata. Below the oesophagus there 

 is generally a small ganglion, connected with the apparatus of deglutition, 

 which may be called the stomato- gastric ganglion. In connexion with the 

 gills we have always one ganglion, sometimes a pair, which may be termed 

 the branchial ganglion. Another is found at the base of the foot, which may 



