Having thus shewn the peculiar organisation of the group he turned to seek out its allies among other families. The Medusæ consisted essentially of two membranes inclosing a variously shaped cavity inasmuch as all its organs were so composed. The generative organs were external, being variously developed processes of the two membranes. The peculiar organs called thread-cells--poisoned darts by the discharge of which prey could be paralysed--were universally present. What other families presented these peculiarities? There are to be found abundantly in sea-water, and less frequently in fresh water, innumerable forms of animal life called Zoöphytes or animal plants because they occur as encrusting masses like lichens, or branched forests like moss, on the surface of stones and shells. A common habit gave this set of creatures their common name; but, although they were grouped together, there was no greater affinity among them than there is racial affinity among people who clothe themselves for an evening party in the same conventional dress. Huxley examined a large number of these, and picked out from them two great families of polyps, the Hydroid and Sertularian polyps, which each consist of colonies of creatures very much like the little fresh-water hydra. He shewed that the tubular body of these and the ring of tentacles surrounding the mouth were composed of the same two foundation-membranes of which all the organs of Medusæ are composed. He found in them the poisoned arrows or thread-cells of the Medusæ, and the same external position of the reproductive organs. And, lastly, he separated from all other creatures, and associated with his new group, some of the strangest and most beautiful animals of the tropic seas, known to science as the Physophoridæ and the Diphyidæ. The best-known of these is the "Portuguese man-of-war," the body of which consists of a large pear-shaped vesicle which floats on the water like a bladder. From the lower part of this depend into the water large and small nutritive branches, each ending in a mouth surrounded by a circle of waving tentacles armed with batteries of thread-cells, while another set of hanging protrusions bear the grape-like reproductive organs. On the upper surface of the bladder is fixed a purple sail of the most brilliant colour, by which the floating creature is blown through the water. When the weather is rough, the bladder empties, and the creature sinks down into the quiet water below the waves, to rise again when the storm is over. This, and its equally wonderful allies, Huxley showed to be a complicated colony of hydra-like creatures, each part being composed of two membranes, and therefore essentially similar to Medusæ. Thus, by a great piece of constructive work, an assemblage of animals was gathered into a new group and shewn to be organised upon one simple and uniform plan, and, even in the most complex and aberrant forms, reducible to the same type.