The ova are probably fertilised in the sea-water; they undergo the early stages of their development whilst entangled amongst the tentacles of the parent. The larval form to which they give rise was known long before its connexion with the adult was demonstrated by Kowalevsky.[[497]] It is known as the Actinotrocha (Fig. 231, A), and according to Caldwell has the following structure in Ph. kowalevskii. The mouth is anterior, and the anus terminal and posterior; the mouth is overhung by an immense prae-oral lobe, which bears a special larval nerve ganglion, and in some species four eye-spots; at the base of this, but behind the mouth, is a ring of larval tentacles. The prae-oral lobe and the tentacles are ciliated; the margin of the lobe bears, however, specially long cilia, and there is also a ring of long cilia around the anus.

Before the Actinotrocha stage has been reached the larva has forsaken the shelter of its parent's tentacles, and swims actively about in the open sea. As it grows older a finger-like involution of the skin (c) arises just behind the tentacles on the ventral surface and grows into the body, increasing greatly in length and becoming much folded. The larva now sinks to the bottom of the sea, and after swimming round many times on its axis, undergoes a very astonishing metamorphosis (Fig. 231, B, C). The finger-like involution is suddenly turned inside out, and forms a large projection on the ventral surface, into which the alimentary canal passes, assuming a U-shape, as in the adult. This ventral process in fact forms all the body of the adult behind the line of tentacles, and subsequently contains, not only the alimentary canal, but the kidneys, the reproductive organs, and a large part of the vascular system. At the same time the prae-oral lobe breaks off, and, together with its ganglia and eye-spots, passes into the mouth and is digested in the stomach; the larval tentacles follow the prae-oral lobe, and are similarly digested. Their place is taken by a ring of adult tentacles which commence to appear just behind the larval tentacles before they fall off. The animal is now practically adult.[[498]]

It is obvious that this astonishing metamorphosis is accompanied by the rotation of the axes of the animal. The adult practically lives at right angles to the larva. In the latter the anus marked the posterior end, and the prae-oral lobe the anterior. The prae-oral lobe has disappeared in the adult, but its position is marked by the mouth. The ventral surface has enormously increased, and corresponds with the whole surface of the trunk. To be consistent we must therefore regard the mouth of the adult as marking the anterior end of the animal, the anus the posterior. The short line between the mouth and anus across the centre of the tentacular crown marks the dorsal surface; and the line running all round the trunk from anus to mouth, the ventral. In fact, in its usual position in its tube Phoronis is lying on its ventral surface, its back faces upwards, and the anterior and posterior ends lie on one side or the other.

Species and Affinities.—In his exhaustive memoir on the anatomy and histology of Phoronis, Cori enumerates seven different species, and quotes the characters of each as enumerated by eight different authors. He, however, reserves his opinion as to the identity or distinctness of some of these species. Benham in his account of Ph. australis enumerates five species, including amongst them Ph. ovalis, which, however, he regards as probably a young form, an opinion in which Cori coincides. The latter regards it as possibly a young form of Ph. hippocrepia.

Without comparing specimens of each of the alleged species, it is difficult to come to any very satisfactory solution of the problem of how many distinct species are at present known, but it seems probable that there are at least six.

(i.) Phoronis hippocrepia Wright.—Under this name is included the first form, described and named by Wright in 1856; also Ph. ovalis, described two years later by the same observer as a distinct form, though it now seems probable that it is but a young form of Ph. hippocrepia. The Crepina gracilis of van Beneden is probably identical with this species.

This species occurs in membranous tubes embedded in limestone, corals, or oyster shells. Its length varies from 1.5 to 15 mm. The number of tentacles varies from 16 to 86. It has been found off the coast of Devonshire and in the Firth of Forth.

(ii.) Phoronis kowalevskii Caldwell.—This name is given by Benham to the species from Naples described by Caldwell, and replaces the name Ph. caespitosa, which was given by Cori. This species is found in the Bay of Naples, living in considerable colonies on submarine piles and posts. It is not firmly attached to its substratum. The tube may be coated with sand or other foreign particles. The length of the individuals varies from 3 to 39 mm. The lophophore is simple, with from 50 to 100 tentacles.

(iii.) Phoronis australis Haswell.—This is the giant of the genus, the length of the individuals being from 3 to 5 (76-127 mm.) or rarely 6 inches. It lives in delicate transparent tubes, interlacing the walls of the tube of a sea-anemone, Cerianthus. The arms of the lophophore coil into two spirals. The colour is reddish or purple. Found in Port Jackson.

(iv.) Phoronis buskii M‘Intosh.—This species was dredged by the Challenger from a sandy bottom at a depth of 10 to 20 fathoms off the Philippines. Its tube is covered with particles of sand, sponge spicules, etc. Its length is 52 mm. or more (more than two inches). The anatomy of this species closely resembles that of Ph. australis, and Benham thinks that, in spite of the difference in their habitat, they may belong to the same species.