Fig. 229.—The dorsal surface of Ph. australis Has., looking down on the head. The tentacles are cut away on the left side, and the innermost are shortened on the right side to show the arrangement; in reality they are of the same length throughout. a, Mouth; b, anus; c, pore of left nephridium; d, epistome; e, break in the inner series of tentacles. The drawing is to some extent diagrammatic, and is considerably enlarged. (After Benham.)
One of the most interesting features of Phoronis is the presence of a closed system of blood-vessels containing red blood. There are two main blood-vessels; one, lying in the rectal chamber between the two limbs of the U-shaped alimentary canal, has been named the afferent vessel. Just below the diaphragm this splits into two, and each branch, after piercing this partition, runs in a spiral course along the base of the crown of tentacles, giving off a single blood-vessel into each tentacle. At its base each tentacular vessel opens not only into the above-mentioned "distributing" vessel, but also into a "recipient" vessel which takes a course parallel with the former. The two recipient vessels pierce the diaphragm, and after running for some distance apart, fuse to form the efferent vessel, which continues down the body on the left side of the oesophagus. At the aboral end of the body the efferent vessel turns forward and becomes the afferent. Both the main vessels give off numerous blood diverticula, which are developed into plexiform sinuses on the walls of the stomach, and in this region they are covered with the reproductive cells. All the vessels are contractile, and Strethill Wright counted about fifteen pulsations a minute. The blood contains numerous nucleated, disc-shaped corpuscles differing in appearance from those of the fluid in the body-cavity. The corpuscles contain haemoglobin, which gives the red colour to the blood.
The two nephridia or kidneys are essentially tubes which open on the one side into the body-cavity, and on the other to the exterior. The position of the external pores has already been described, one being on each side of the anus. Each pore leads into a tube which passes into that part of the body-cavity situated below the diaphragm, where it divides, and each of the two branches terminates in a ciliated funnel-shaped opening. The smaller of these two funnels pierces the lateral mesentery and opens into the lateral chamber, whilst the larger, whose opening is very much drawn out longitudinally, opens into the rectal chamber. The whole organ is ciliated internally.
The nervous system lies in the skin immediately below the epidermis. This position is very primitive, and forms one of the most interesting anatomical peculiarities of the genus. The nervous tissue is probably diffused all over the body, but there is a special concentration or thickening in the form of a ring which surrounds the mouth, following the base of the tentacular spirals and giving off a nerve to each tentacle. The ring lies at the outside of the base of the tentacles, the anus is not included in it. Caldwell[[496]] has described in Ph. kowalevskii an asymmetrical nerve-cord given off from the ring and running along the left side of the body; associated with which is a tubular structure of unknown function. In Ph. australis Benham mentions two such tubes, one on each side of the body; their precise value is obscure.
The epithelium covering the nerve-ring is slightly modified in the neighbourhood of the kidney pore, and may have some special sensory function; no other organs of sense are known (but see p. [454]).
Fig. 230.—A schematic view of the interior of the body of Phoronis. The middle seven-eighths of the body are omitted, af, Afferent blood-vessel; an, anus; ef, efferent blood-vessel; ep, epistome; gl, glandular pit; int, intestine or "second stomach"; k, large funnel of the left nephridium; ko, opening of right nephridium, the opening of the left is seen immediately below in section; m, mouth; n, nerve concentration; oe, oesophagus; oem, oesophageal mesentery; ov, ovary; r, rectum; rm, rectal mesentery; rv, right recipient blood-vessel; s, septum; st, stomach; t, testis; tm, right lateral mesentery. (From Benham.)
Phoronis is hermaphrodite, male and female reproductive cells being formed in the same individual. The testes and ovaries form two white masses lying on the left side of the stomach, one on one side and the other on the other side of the efferent blood-vessel. The glands are traversed in all directions by the diverticula given off from this trunk, and are thus well supplied with blood; in fact both the ovary and the testis are formed by the multiplication and growth of the epithelial cells which cover these diverticula. When ripe the ova and spermatozoa drop off into the body-cavity and make their way to the exterior through the duct of the kidney.
Fig. 231.—Three stages in the metamorphosis of the Actinotrocha into Phoronis. A, Actinotrocha larva with the invagination (c), which will form the trunk of the Phoronis larva beginning to appear. B, Stage with the invagination partly extruded. C, Stage when the extrusion is complete and the alimentary canal has passed into it. C is after Metschnikoff. a, Mouth; b, anus; c, invagination which ultimately forms the greater part of the body of the adult.