In earlier experiments (Davenport. 1950), it was demonstrated that the specificity of the commensal polynoid worms Arctonoc fragilis (Baird) and A. pnlchra (Johnson), respectively inhabiting the starfish Evasterias troschelii Stimpson and the sea-cucumber Sticlwpiis californicus (Stimpson), could at least in part be explained by a positive response which these commensals showed to their hosts. On the basis of these tests it was concluded that "certain echinoderm hosts produce a specific diffusible substance that acts as a powerful attractant for their commensals." Welsh (1930Welsh ( , 1931 had demonstrated that similar agents govern the specificity of certain clam-acarine partnerships. In the course of his work these chemical agents were found to be relatively stable. Boiling for five to ten minutes as well as putrefaction for a week at 37.5 C. had little effect on the material acting on the mites. Beyond this, no further investigation of the nature of the substance was carried out.
tory) remain in the plankton for some 30 days, nevertheless only successful breeding and settling experiments will give us the answer. SUMMARY 1 . A new water-table test apparatus for the investigation of commensal response behavior is described.
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