This interesting genus was discovered and first described by Dr. Strethill Wright of Edinburgh, who in the year 1856 found specimens of it living on a stone with Caryophyllia sent to him from Ilfracombe. He christened the form Phoronis hippocrepia,[[490]] the generic name being apparently taken from an epithet applied to Io, the specific name having reference to the beautiful horseshoe shape of its tentacular crown. Two years later a closely allied or identical form was described by Professor P. J. van Beneden under the name of Crepina gracilis.[[491]]
Phoronis is a sedentary animal living in "colonies," but each member of the colony is distinct, and has no organic connexion with the others, from which it is isolated by the presence of a tube in which it lives, and into which it can be completely withdrawn. The tube is formed from a secretion which probably has its origin from the anterior end of the body-wall. The secretion hardens and forms at first a transparent coating, but it soon becomes opaque, and numerous sand particles, small pieces of shell, sponge spicules, and other marine objects adhere to the outside of the tubes, giving them a very characteristic appearance, and doubtless serving to protect the inhabitants from predatory animals.
What little we know about the habits of Phoronis is in the main due to the observations of Cori,[[492]] who studied Ph. psammophila at Faro, an inlet of the sea near Messina. The least disturbance causes the animal to withdraw its head with lightning rapidity into the tube, from which after a time it re-emerges very slowly, and does not expand its tentacular crown until its body is completely extended. Cori states that not unfrequently individuals are found either without the crown of tentacles or with the latter in process of regeneration. These may have been bitten off by fish, etc.; but, on the other hand, van Beneden describes in Crepina gracilis (Ph. hippocrepia) the throwing off and regeneration of the crown of tentacles; and Cori confirms his observation, at any rate as far as concerns those individuals kept in captivity, and whose surroundings were presumably somewhat unfavourable. He further observed the interesting fact that the cast-off crown of tentacles continued to live, and suggests that possibly it may develop a new body, in which case the phenomenon would be an interesting case of binary fission producing two new animals.
Fig. 226.—A piece of a matted colony of Ph. kowalevskii Cald. Slightly magnified. In most cases the tentacular head is protruding from the tube.
With regard to the habitation of Ph. australis, the largest species known, some discrepancies have crept into the literature of the genus, and to prevent their recurring again it may be worth while to quote the statements of its discoverer, Mr. Haswell.[[493]] He says: "Phoronis australis occurs in communities of twenty to thirty, in spaces in the substance of the wall of the tube inhabited and formed by a species of Cerianthus. Each worm has a tube of its own, very delicate and transparent, made up of several layers, the mouth opening on the outer surface of the tube of the Cerianthus. The Cerianthus tubes sometimes come up empty, as we should naturally expect, the animal having dropped out; but a sufficient number of occupied tubes are found to show that, under ordinary circumstances, a living Cerianthus occupies the interior of the tube and a community of Phoronis live in its wall. This species of Phoronis is never found anywhere else, and the species of Cerianthus is very rarely found without the Phoronis."
Ph. australis is sluggish in its movements, but other species are capable of very active movement, and withdraw their heads in a moment at the approach of danger. A Neapolitan species, Ph. kowalevskii—known to the fishermen of that place as "Ficchetelli bianchi" or "Vermi di ceppa"—lives chiefly on submarine posts and piles; its tubes, closely interlacing, form a dense feltwork, upon which Ascidians and Sea-anemones often settle, and over which Ophiurids and Polychaets creep. The tubes of this species are rendered opaque by the excreta ejected from the body, and they do not attach foreign substances to the outside to anything like the same degree as Ph. psammophila, which live in sandy places, and are termed by the Sicilian fishermen "Tubi di sabbia." The feltwork of Ph. kowalevskii attains a thickness of 5 to 8 cm. In each case the tube is much longer than the animal it shelters, and is so entangled with its neighbours, to which it frequently adheres, that it is a matter of considerable difficulty to isolate it.
Fig. 227.—A piece of a colony of Ph. psammophila Cori. Slightly magnified. The tubes are covered by particles of sand, small shells, etc.
The various species of Phoronis differ a good deal in size; Cori gives the average length as varying from 1.5 to 7.9 mm. in Ph. hippocrepia and up to 127 mm. (6 inches) in Ph. australis. Probably the very short individuals of the first-named species had not attained their adult stature. Ph. australis has recently formed the subject of a memoir by Dr. W. B. Benham,[[494]] from whom the following account is mainly taken.