The colour-sense: its origin and development : an essay in comparative psychology / by Grant Allen.

  • Grant Allen
Date:
1892
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Licence: Public Domain Mark

Credit: The colour-sense: its origin and development : an essay in comparative psychology / by Grant Allen. Source: Wellcome Collection.

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    acquire their tentacles and assume the free mode of life, the ocelli are produced together with the other mature organs. This must he regarded as a case of progressive development. If we examine the various classes of animals in order, the same general connection between free locomotion and vision will be forced upon us once more. Passing over the Protozoa, which of course are too humble in structure to exhibit any such complex nervous organs as eyes, and beginning with the Eadiata, we see that the only class in that division which possesses high powers of locomotion is the Discophora, or jellyfish, and this is also the only class provided with visual organs. Among the Kemato- phora or Echinodermata, which are all very sedentary animals, eyes are doubtfully present. The lower vermi- form Articulata are mostly entozoic, and these of course are quite blind; but the few species which swim freely in water by means of cilia have eyes with distinct lenses. The free leeches have a ring of eyes around the sucking- disc. The highest of these vermiform creatures—the Nereidce, Peripatidm, and Polyophthalmidm—are all very locomotive, and all have very highly developed organs of vision. So likewise have the active little Eotifera. The Arthropoda, or true articulates, yield like results. Thus, among the Crustacea, the Cirrhopoda in their fixed state and the parasitic Entomostraca are sightless ; but all the higher free crustaceans are provided with eyes, which in the active crab and lobster orders attain a high degree of perfection. The flying insects show us eyes of great com- plexity, inferior only to the same organs in vertebrates, if even to those. Yet while most of the Hymenoptera (in- cluding the wasps and bees) have very acute vision, it is noteworthy that the ants, which have practically lost their wings, are almost, and in some species quite, blind. It is also a remarkable fact that the male and female ants which are winged possess three ocelli, wanting in the wingless neuters. Among the Mollusca, in like maimer,