The pre-fertilization behavior of the sperm of the thecate hydroids Campanularia flexuosa and Campanularia calceolifera has been observed and recorded by means of dark-field cinephotomicrography. The sperm of both species activate and aggregate homotypically around the aperture of the female gonangium. Plots of the paths of sperm approaching the aperture show that the aggregations are the result of directed turning movements and not of so-called trap-actions or an increased random turning of the spermatozoa. An aperture-associated tissue of ectoderm origin has been found to produce the attractive substances.

Active sea water and alcohol extracts of female gonangia of C. calceolifera have been prepared. The substance affects sperm in the same ways as the material released by the female gonangium. The active material is heat stable, non-volatile, dialyzable, polar, and appears to be a single molecular species of less than 5,000 mw. Injection of these extracts into sperm suspensions by means of a micropipette produces activation and aggregation of only the sperm of C. calceolifera about the pipette tip. If agar impregnated with the chemotactant is used as a source, the sperm are seen to orient with considerable accuracy. Extraction of C. flexuosa female gonangia has not yielded active extracts.

These results demonstrate that chemotaxis does occur during fertilization in an animal. The mechanism of the reaction remains to be determined.