182 NERVOUS SYSTEM. 



water; they contract and bend their body in every direction, and 

 they increase or retard, or cease at pleasure, their progressive 

 motion and the vibration of their cilia, like the muscular and 

 gangliated rotiferous animalcules, yet nervous filaments have 

 not been distinctly detected in the minute transparent bodies 

 of the polygastrica. The numerous straight parallel jaws, 

 seen in many of the genera, are opened and closed, advanced 

 and retracted, with great quickness and precision, and all the 

 movements of these minute animals appear to be as regular, 

 methodical, spontaneous, and well-directed as those of many 

 higher animals with obvious nerves. The transparency of 

 the nervous filaments in all the minute forms of animals 

 probably prevents our detecting in the polygastrica the rudi- 

 ment of that form of the nervous system which is seen in 

 the wheel-animalcules, and in the higher articulated classes. 

 In the poriphera, the component particles of the nervous 

 and muscular systems are probably diffused through every 

 part of the soft cellular tissue of the body, which possesses 

 the same living properties in every part, and is almost inde- 

 finitely divisible without destroying its vitality. The repro- 

 ductive gemmules of these animals vibrate their cilia with 

 great regularity and force ; they appear to be conscious of 

 each other's approach, and can accelerate, retard, or cease 

 their motions at pleasure ; they are sensitive to light, and 

 appear to be guided by its influence in selecting the place of 

 attachment most suited for the growth of each species : yet 

 they exhibit no nervous or muscular filament in the gela- 

 tinous texture of their body. Many polypipherous animals, 

 even of the simplest forms, are obviously sensitive to light, 

 as hydra, lobularite, actiniae, and muscular fibres are distinctly 

 perceptible in the polypi and other parts of most of the 

 higher genera. The nervous system has been long known in 

 the actinia, which is a large isolated naked polypus, closely 

 resembling, in external form and internal structure, the po- 

 lypi of caryophillice, pavonite, and many of the larger litho- 

 phytes. Nervous filaments surround the muscular foot of 

 the actinia, beneath the stomach, and present minute gan- 

 glia in their course, from which nerves pass out to the cir- 

 cumference, and to the muscular folds which here possess 

 great power of contraction. The same system probably ex- 

 ists in many other closely allied forms of polypi. The lowest 



