The Symbionts

As the epochs passed, the two species molded one another to form a well-integrated union. The little arachnoid, no bigger than a chimpanzee, rode in a snug hollow behind the great "fish's" skull, his back being stream-lined with the contours of the larger creature. The tentacles of the ichthyoid were specialized for large-scale manipulation, those of the arachnoid for minute work. A biochemical interdependence also evolved. Through a membrane in the ichthyoid's pouch an exchange of endocrine products took place. The mechanism enabled the arachnoid to become fully aquatic. So long as it had frequent contact with its host, it could stay under water for any length of time and descend to any depth. A striking mental adaptation also occurred in the two species. The ichthyoids became on the whole more introvert, the arachnoids more extrovert.

Up to puberty the young of both species were free-living individuals; but, as their symbiotic organization developed, each sought out a partner of the opposite species. The union which followed was life-long, and was interrupted only by brief sexual matings. The symbiosis itself constituted a kind of contrapuntal sexuality; but a sexuality that was purely mental, since, of course, for copulation and reproduction each individual had to seek out a partner belonging to his or her own species. We found, however, that even the symbiotic partnership consisted invariably of a male of one species and a female of the other; and the male, whichever his species, behaved with parental devotion to the young of his symbiotic partner.