The interest of Phoronis lies in its strong resemblance to cer tain Polyzoa (q.v.). Lankester went so far as to call Phoronis the only solitary polyzoon. But Phoronis possesses, distinct alto gether from the body-cavity, a well-developed blood-system, con taining a colourless serum in which float red blood corpuscles. No such organ is found in any polyzoon. This blood-system consists of an "artery" running on the anal side of the gullet to the level of the lophophore where it gives off right and left a vessel to each arm of the lophophore. From these a single vessel goes to each tentacle. This vessel at the base of the tentacle sends a fork to an outer or recipient vessel, which runs along the convex side of the lophophore on each side. The right and left recipient vessels pass downwards, pierce the lophophoral platform and unite on the convex side of the oesophagus to form a single "vein" which below the stomach opens into the artery.
The reproduction raises other questions. The eggs, after being shed, stick to the tentacles and are there fertilized by sperms from another individual. They complete the first stages of their devel opment within the egg-membrane, from which they hatch out as peculiar free-swimming larvae known as Actinotrocha. This larva is provided with a ciliated forehead in which a large ganglion is embedded. There is an oblique wreath of ciliated tentacles behind
the mouth by which it swims. There is a belt of cilia round the hind end of the body in front of the anus. This larva after feeding for a time and growing greatly in size, develops an intucking of skin on its under-surface, to which a fold of the intestine becomes attached. Suddenly (inside a quarter of an hour) this intucked area is everted, forming a finger-shaped process containing a loop of the intestine which gives rise to the body of the "worm." The ciliated forehead is cast off, as are also the ciliated tentacles, but new tentacles which give rise to the lophophore of the adult are regenerated from their stumps. This larva with its extraordinary life-history has given rise to much speculation. We owe the clear ing up of its real affinities to E. S. Goodrich and Shearer, who have shown that Actinotrocha is merely a modified trochophore, such as occurs in the life-history of the Gephyrea amongst the Annelida (q.v.). The typical trochophore has a broad preoral belt of cilia, the prototroch, and a slender postoral one, the metatroch, whilst a belt of cilia, the telotroch, encircles just in front of the anus.
W. MAcB.)