Fam. 1. Edwardsiidae.—Several species of this family have been found in the British area. They are very local in their distribution, but sometimes occur in great numbers.
Edwardsia beautempsii occurs in shallow water near the shores of the English Channel and has been found in Bantry Bay; and E. carnea and E. timida have also been found in the Channel. E. tecta is a recently described species from the S. Irish coast, and E. allmani and E. goodsiri are found in Scottish waters.
Fam. 2. Protantheidae.—This family, constituted for the reception of three remarkable genera, is now usually included in the order Edwardsiidea on the ground that not more than eight mesenteries are complete.
The genus Gonactinia exhibits the very exceptional character of having a thick layer of muscles in the body-wall (cf. Cerianthidea, p. [409]), and it is also remarkable for the frequency with which it reproduces itself asexually by longitudinal and, more rarely, by transverse fission. It has been found in Norway, the Mediterranean, and on the reefs of New Caledonia. The other genera of the family are Oractis from California, and Protanthea from the coast of Sweden.
Order II. Actiniaria.
This order contains nearly all the animals popularly known as Sea-anemones. They are usually found in shallow water, attached by a broad basal disc to shells, stones, or sea-weeds. In the Halcampidae, however, the aboral extremity ends in a blunt point as in the Cerianthidea and Edwardsiidea, and the animals live half-buried in sand or mud. The Minyadidae of the southern oceans are pelagic in habit, floating near the surface of the sea with the mouth turned downwards. They are supported in the water by a bladder, formed by an involution of the pedal disc, and filled with gas.
Many of the Sea-anemones are found in symbiotic association with other animals. The common Adamsia of the British coasts is found on whelk shells containing hermit crabs. The crab is probably protected from the attacks of some of its enemies by the presence of the Anemone, which in its turn has the advantage of securing some fragments of the food captured and torn to pieces by the crab. The association, therefore, seems to be one of mutual advantage to the messmates. It is a noteworthy fact that in these associations the species of Sea-anemone associated with a particular hermit crab is nearly always constant. Thus in the English Channel, Adamsia palliata is almost invariably found associated with Eupagurus prideauxii, and Adamsia rondeletii with Eupagurus bernhardus. But, perhaps, the most remarkable association of this kind is to be seen in the case of the little shore crab of the Indian Ocean, Melia tesselata, which invariably holds in each of its large claws a small Sea-anemone. Möbius, who originally described this case, relates that when the crab is robbed of its Anemone it appears to be greatly agitated, and hunts about on the sand in the endeavour to find it again, and will even collect the pieces, if the Anemone is cut up, and arrange them in its claw.[[400]]
Another very interesting association is that of certain fish and Crustacea with the large Sea-anemones of the tropical Australian coast.[[401]] Thus Stoichactis kenti almost invariably contains two or more specimens of the Percoid fish Amphiprion percula. This fish is remarkable for its brilliant colour, three pearly white cross-bands interrupt a ground plan of bright orange-vermilion, and the ends of the cross-bands as well as the fins are bordered with black. In another species a prawn of similar striking colours is found. These companions of the giant Anemones swim about among the tentacles unharmed, and when disturbed seek refuge in the mouth. It has been suggested that these bright and attractive animals serve as a lure or bait for other animals, which are enticed into striking distance of the stinging threads of the Anemone, but how the commensals escape the fate of the animals they attract has yet to be explained.
In a considerable number of Sea-anemones, such as Actinoloba marginata and A. dianthus, some species of Sagartia, Actinia cari, Anemonia sulcata, and Calliactis parasitica, the fertilisation of the eggs and their subsequent development take place in the sea water.[[402]] In a great many others, such as Bunodes (several species), Cereactis aurantiaca, Sagartia troglodytes, Bunodactis gemmacea, etc., the embryos are discharged into the water from the body-cavity of the parent, at a stage with six or twelve tentacles. In the Arctic species of the genera Urticina and Actinostola, however, the embryos are retained within the body of the parent until several cycles of tentacles are developed, and in Urticina crassicornis the young have been found with the full number of tentacles already formed. In Epiactis prolifera from Puget Sound, the young Anemones attach themselves to the body-wall of the parent after their discharge, and in Epiactis marsupialis, Pseudophellia arctica, Epigonactis fecunda, and other species from cold waters, the young are found in numerous brood sacs opening in rows on the body-wall. It is not known for certain how these embryos enter the brood sacs, but it is possible that each sac is formed independently for a young embryo that has settled down from the outside upon the body-wall of the parent. The most specialised example of this kind of parental care in the Sea-anemones is seen in Marsupifer valdiviae from Kerguelen, in which there are only six brood sacs, but each one contains a great many (50-100) embryos.
The wonderful colours of our British Sea-anemones are familiar to most persons who have visited the sea-side. The common Actinia mesembryanthemum of rock pools, for example, is of a purple red colour. The base is usually green with an azure line. Around the margin of the disc there are some twenty-five turquoise blue tubercles. On each side of the mouth there is a small purple spot, and the numerous tentacles forming a circlet round the mouth are of a pale roseate colour. Nothing could be more beautiful than the snowy-white Actinoloba dianthus or the variegated Urticina crassicornis.