I read that compost worms dislike light. But I'd say they don't have eyes and I couldn't imagine their skin is sensitive to light in general, is it? And if so, then why?

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    Making a quick guess I would say that Sunlight would dry them out fairly quickly as they do not have the dead layer of keratinized cells on their surface that protects them as mammals do. Also UV light will damage their DNA pretty rapidly, again for similar reasons, so keeping in the dark and having a photosensitive response to sunlight would likely have been an evolutionarily advantageous strategy for the niche they inhabit. – AMR Nov 18 '15 at 14:01
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    And yes, cells, even unicellular organism, can be sensitive to light and can move as a result of those sensations. Aquatic Microorganisms will often adjust the depth they are at in water as a result of the presence or absence of light. Remember that a photon will impart its energy on electrons in the cell and excited electrons can cause chemical reactions to take place and these reactions can stimulate the organism to move. – AMR Nov 18 '15 at 14:06
up vote 3 down vote accepted

It will sounds counter-intuitive you but earthworms do sense light with their skin. They don't have eyes but they have specialized nerve ending equipped with photoreceptor cells. They are all over skin but concentrated towards first segment of worm. Prostomiun are main light sensing organ in worms. Following image shows their position in first segment of earthworm. (Image source: cronodon.com). enter image description here

Closer look of above image taken from same website,

enter image description here

Structure of photoreceptors

Earthworm have microvillar photoreceptors like insects (Rohlich et al 1970). Detailed structure is discovered long back. enter image description here

There is not much information about phototrasnduction in worms but it is well characterized in it's close relative C. elegans (Ward et al 2007).

Now coming to why part, you might wonder if they are spending most of time buried why do they need photoreceptors ? @AMR has guessed it correctly. They use this as protective system. Photoreceptors helps them providing information about how near they are to surface because UV radiation will harm them or probably kill. If there is higher light sensing, they go back to darker regions. You can find similar information from UMICH press release.

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