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Are there any examples of brainless animals (e.g. jellyfish) exhibiting seeking behavior, such as following smell or light gradients towards food, or following hormones towards mates?

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Yes, there are plenty of examples! We refer to this seeking behaviour as taxis.

Taxis

There are many types of taxis such as chemotaxis (attracted to chemical stimulus) or phototaxis (attracted to light). A complete list of taxis can be found on wikipedia > taxis.

For the purpose of this answer I will restrict to chemotaxis only

Chemotaxis (from chemo- + taxis) is the movement of an organism in response to a chemical stimulus.

Chemotaxis in Bacteria

Chemotaxis is very common in bacteria such as E. coli for example (Macnab and Koshland 1972, Wadhams and Armitag 2004).

Chemotaxis in Plants

Chemotaxis is also observed in plants such as rhibozium spp (Currier and Strobel 1976) for example

Chemotaxis in eukaryotes

Chemotaxis is also found in eukaryotes such as in Amoeba or Tetrahymena (Köhidai 1999,Bagorda and Parent 2008) for examples.

Chemotaxis in animals that have "primitive" brains

Nematodes (Ward 1973) and plathelminthes (Bone 2012) for example show chemotaxis.

Chemotaxis in "brainless" animals

Evidence of chemotaxis also come from Echinodermata (sea stars, sea urchins and others) (Miller 1985).

Echinodermata have quite definitely no brain!

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