This group contains only a single marine genus, Phoronis, with several species. It occurs on our own coasts in societies of separate individuals. The animal inhabits a fixed leathery tube within which it can move. It has a long body or stem, really an out-growth of the ventral surface, the true dorsum being represented by a short line joining the mouth and anus. The latter lies in the centre of the concavity of a horseshoe-shaped disc or lophophore, which bears a series of ciliated tentacles supported by a mesoblastic skeleton, and surrounded at the base like that of Phylactolaematous Polyzoa by a calyx or thin membrane. The youngest tentacles are the two nearest the dorsal median line, in the concavity of the lophophore. The mouth is overhung by a valve or epistome, the remains of the larval praeoral lobe. The nervous system lies in the ectoderm. It is concentrated in two places; as a circumoral ring following the line of the tentacles, and as a cord which runs down the left side of the body. Two ciliated sensory pits lie one on either side the anus. The alimentary canal is ciliated and divisible into an oesophagus, a first stomach, a second stomach, which is a 'small strongly ciliated chamber at the spot where the canal bends on itself in the body/ and an intestine.
The coelome, which is an archenteron, is divided into an anterior and posterior part by a septum which crosses the lophophore from one side to the other at the level of the nerve ring. The anterior part is therefore small. The posterior is subdivided into three longitudinal chambers, which communicate at the base of the body, by three mesenteries, one median connecting the outer aspect of the digestive tract in its whole extent to the body-wall, the other two extending laterally from the stomach only to the body-wall. There is a vascular system consisting of two longitudinal trunks, one of which divides in the median dorsal region, and forms a ring lying at the base of the tentacles into each of which a caecal vessel passes. An outer ring is connected to these same vessels by valved inlets. It divides; the branches surround the oesophagus, and unite to form a second, the longitudinal ventral vessel, which runs towards the base of the body in the left ventral chamber of the coelome giving off many lateral caecal branches. There is a vascular sinus round the stomach. The vessels are all contractile and contain nucleated corpuscles tinted with haemoglobin (or haemerythrin ?). A fatty tissue surrounds the lateral caecal vessels above named.
In it are developed, but on opposite sides, the spermatozoa and ova. The latter are impregnated in the coelome, and pass out thence by two ciliated tubes or nephridia, which open internally into the single one of the three posterior chambers of the coelome, and externally near the anus. The ova adhere to the tentacles. Segmentation is total, but unequal. The larger cells are invaginated, and the blastopore persists as the mouth. The larva is known as Actinotrocha. It has a large praeoral hood-like lobe with strongly ciliated edges: an imperfect postoral circlet of ciliated larval tentacles: and a ring of large cilia round the anus which is a proctodaeum. The body is also ciliated. There is a supra-oesophageal ganglion in the praeoral lobe, a sense organ in some species, and in one, four eye-spots. At the metamorphosis the praeoral lobe, ganglion, and larval tentacles, are swallowed and digested after disruption along a line of vacuolated ectoderm cells (Caldwell). The body, at first developed as a ventral invagination, is suddenly evaginated, and a new set of tentacles is formed.
Caldwell, P. R. S. xxxiv. 1883; Ray Lankester, Encyclopaedia Britannica (td. ix), xix. p. 433.
For Actinotrocha, see also Wilson, Q. J. M. xxi. 1881.