We need not, I think, assume that every case of commensalism amongst animals has had a utilitarian origin, even when we can now see the link of mutual benefit.
The distinction between symbiosis and commensalism cannot be rigid, but between these conditions which are advantageous to the partners and parasitism, there is an obvious and radical difference.
Symbiosis and commensalism must be distinguished from parasitism, which indicates that all the advantage is on the side of the parasite, and nothing but loss on the side of the host.
Remarkable cases occur of commensalism between certain crabs and sea-anemones, and they betoken much intelligence.
Cases of commensalismamong the Echinodermata are still more rare.
Professor Allman has just observed a very remarkable case of commensalism between a sponge and one of the tubulariae.
Another kind ofcommensalism is that of the Dromiae.
Animal messmates are rather numerous, and commensalism has been observed, not only in animals of the present age, but in those of the primary epoch.
It is to the latter that we refer here, and it seems that in this species we are approaching the limits at which commensalism commences.
But the precise limit at which commensalism begins is not always easily to be ascertained.
