File:The pedigree of man - and other essays (1903) (14764434742).jpg

Identifier: pedigreeofmanoth00haec (find matches)
Title: The pedigree of man : and other essays
Year: 1903 (1900s)
Authors: Haeckel, Ernst Heinrich Philipp August, 1834-1919
Subjects: Man Evolution Man Evolution
Publisher: London : A and H.B. Bonner
Contributing Library: ASC - York University Libraries
Digitizing Sponsor: York University - University of Toronto Libraries

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is. Theexternal integument is the original, general sense-organ. Gradually the higher sense-organs detach them-selves from this their primal condition, whilst theywithdraw more or less completely into the protectinginterior of the body. Nevertheless in many animals,even at the present hour, they lie in the integument, ase.g. in the Vermes (Fig. 50). But the activity of the sense-organs, as of all others,depends entirely upon that of the microscopic cellscomposing them. These small cells are, in fact, thetrue, independent elemental organisms, whose func- SENSE ORGANS. tions in their continued totality condition the life ofthe whole multicellular organism. Hence the sense-organs, the sense-cells that give rise to the varioussensory consciousnesses, are of greatest moment: thevisual cells of the eye, the auditory of the ear, theolfactory of the nose, the gustatory of the tongue. If,then, as we now know, all the various sense-organs areactually only particular parts of the integument spe-
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Fig. 50. Nervous system and sense organs of the Turbellaria. Twokinds of nerves radiate from the simple nerve-ganglion orbrain (g) : the centripetal sensory nerves (s) pass to theskin (h), tentacles (£), auditory vesicles (o), eyes (a); thecentrifugal motor-nerves (m) go to the flesh, the sub-mus-cular epidermal layer (/) ; w cilia of the epidermis. cialised and modified, all the various sense-cells musthave originally arisen from single epidermal cells.They are, in fact, as a whole transformed descendants, 304 SENSE ORGANS. specialised in different ways, of the epidermal cells thatare for the most part undifferentiated (Fig. 51). This fundamental fact, whose significance cannot bevalued too highly, is now established beyond a doubt.Every one who studies the incubated egg by the aid of

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