VOLUNTARY MOTION. 501 



They have been discovered in agreat number of entozoa. Even in the actiniae, 

 some medusae, and other zoophyta, muscular fasciculi, interwoven with the 

 external skin, have been perceived ; and Professor Ehrenberg has detected them 

 in the infusoria.* Their fibres are not always red, but may be white, or 

 yellowish. 



Minute and numerous hair-like processes, called cilia, are observed on the ex- 

 ternal surface of batrachian larvas, of mollusca, annelida, echinodermata, actiniae, 

 medusa?, polypi, and infusoria ; on the surface of the air passages of man and 

 other mammalia, birds and reptiles, as well as on the external gills of batrachian 

 larvae, and on the gills of mollusca and annelida ; in other annelida, and in 

 echinodermata and actiniae, on the membrane of the external surface of the 

 viscera and its parietal portion to which water has access ; on the surface of the 

 mouth and gullet of reptiles, and more or less on the whole of the rest of the 

 alimentary canal of mollusca, actinias, annelida, echinodermata, and polypi; in 

 the pores and canals of sponges ; on the mucous membrane of the Fallopian tubes, 

 uterus and vagina of mammalia and fish, as well as on the organ of smell in the 

 latter, and the oviduct of birds and reptiles ; on the surface of the embryo of 

 batrachia, mollusca, actinias, polypi, and sponges. The longest cilia hitherto 

 measured have proved -005 of an inch, the smallest -000075 of an inch. The 

 motion of each cilium is commonly of a fanning character, though sometimes it 

 is rotatory, or, as the point revolves the most extensively, infundibuli-form ; and 

 separation of the part, and even death, does not arrest it for a few hours in 

 mammalia and birds, nor for upwards of a fortnight in the tortoise and river 

 mussel, differences corresponding with the varied duration of muscular con- 

 tractility in the same animals. The purpose of this vibration is to carry the 

 animal through the fluid in which it lives, or to drive fluids along its surfaces. 

 For a full account of the cilia, see a paper by my no less excellent than able 

 colleague Dr. Sharpey, in Dr. Todd's Cyclopaedia of Anatomy and Physiology 

 Very recently Purkinje has discovered them on the linings of the cerebral cavi- 

 ties of the foetuses of some mammalia. Miiller's Archivfiir Anatomie, Physio- 

 logic, &c. No. 1. 183$. p. 291. sq. 



Sometimes, as in Crustacea and insects, they are situated in hollow, calcareous 

 or horny parts ; sometimes in earthy shells, as in bivalve and multivalve mol- 

 lusca. 



A writer says that he " repeatedly placed a common dorr," the occupation 

 of which beetle is to heave up the earth, " under a weight equal to 4796 grains, 

 319 times its own weight," the animal being but 15 grains, and the creature 

 " heaved it up and withdrew ; and the same pressure, being placed on its leg, 

 was immediately disengaged by the power of the other." (Journal of a Naturalist, 

 p. 305.) 



Muscular power is no where more displayed than in some fish. " I have 

 seen," says Sir Gilbert Blane, " the sword of a swordfish sticking in a plank 



* See Dr. Gardiner's account of Professor Ehrenberg's discoveries in the 

 Edin. New Phil. Journ. 1831. 



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