334 SPECIAL PHYSIOLOGY. 



the perfect butterfly or beetle ; and such combining and controlling power, as 

 is necessary to bring the locomotive movements into harmonious action, is, of 

 course, of feebler character ; hence the small relative size of the cephalic gan- 

 glion. In the chrysalis stage, in which preparation is already being made 

 for the development of the perfect insect, suitable modifications in the nervous 

 system begin to take place ; these consist, first, in a concentration of some of 

 the longitudinal chain of ganglia, especially of those corresponding with the 

 thoracic region, now and hereafter to be developed as the basis of support, not 

 only of the lower limbs, but also of the wings, and containing the large mus- 

 cles which move those members ; secondly, in an actual wasting, or disappear- 

 ance, of some of the abdominal ganglia, in accordance with changes, or abor- 

 tions of the segments themselves ; and lastly, in an increase in the size of the 

 cephalic ganglia, harmonizing with the foreshadowed increased development 

 of the sensory powers, of the sensori-motor or instinctive faculty, and of such 

 volitional manifestations, as the perfect insect is capable of. In the perfect 

 insect, these changes in the nervous system attain their full development ; 

 the cephalic ganglion, the great sensory centre, and controlling motor centre, 

 assumes a preponderating size, in comparison with the other ganglia ; the 

 thoracic pairs of ganglia are enlarged and concentrated ; whilst the abdominal 

 ganglia remain small, or even diminish in size posteriorly. 



Annuloida. 



The quadruple, double, or single cephalic ganglion of the vermiform Scole- 

 cida, or Kotiferous animalcules, undoubtedly combines the sensory and the 

 sensori-motor powers, and the reflex faculty as well ; but there is no evidence 

 of real volition in these animals. It has connected with it, the nerves of the 

 tentacles, and those of certain ciliated, and possibly sensory, sacs, and resting 

 upon it, in some cases, are pigmentary eye-spots, or rudimentary ocular 

 organs. The Rotifera, at all events, seem to be attracted by light and heat. 

 Passing backwards from the cephalic ganglionic mass, are also nerves, which 

 supply the skin, the muscles of the body, and even the viscera. The nervous 

 system of the Echinodermata, is very peculiar, being adapted to the radiated 

 condition of the body ; its several ganglia may be regarded as parts of one 

 divided cephalic ganglion connected by commissures ; from its five segments, 

 afferent and efferent fibres proceed to the rays, or divisions, of the animal, to 

 the mouth and viscera ; the eye-spots, found in some star-fishes, also have 

 minute nerves traceable to them. There is certainly no sign of volition in the 

 Echinodermata ; their sensation is also of the lowest order ; and the sensori- 

 motor, or instinctive, movements are probably almost entirely replaced by 

 purely excito-motor, or reflex acts, which would almost explain their habits 

 and life. None of the five ganglia are larger than, or exhibit any superiority 

 over, the rest ; they may be regarded as presenting an example of a subdivided 

 locomotive or Molluscous pedal ganglion, with but a slight share of sensibility 

 superadded. Moreover, the ganglia are connected by some physiological, as 

 well as anatomical, bond ; for, in the Synapta, a species of star-fish, which 

 has the habit of casting off', at its base, an injured or irritated ray, this power 

 is lost, if any part of the nervous circle around the mouth, be previously cut 

 through. 



The functions of the sympathetic system, seem to be performed by the gen- 

 eral nervous system, or by parts blended with it ; for no distinct ganglia ap- 

 pear to be set aside for it. 



Ccdenterata. 



In the few instances, in which a distinct nervous system has been detected in 

 this subkingdom, as in Beroe and Cydippe, its ganglionic central mass has a spot 

 of pigment upon it, and gives off' nerves to the soft contractile walls of the 

 body, and to the so-called auditory sacs. It must fulfil a sympathetic and 

 reflex office ; it may also exercise the feeblest sensory or sensori-motor power ; 

 but no volitional functions. It is possible, that in the simplest forms, such as 

 the Hydra, no nervous system exists ; it is more probable, however, that it is 



