Fig. 12.—Small portion of colony of Rhabdopleura normani, Allman, Lofoten Islands, × 16. a, Anus; p, proboscis (= buccal disc); r, rod-like axis of the adherent part of the colony, prolonged into s, the stalks of the individuals; st, stomach; t, the two tentacular arms of the collar. (After Sars.)
Rhabdopleura differs from Cephalodiscus in its much smaller size,[[48]] and it is perhaps due to its minuteness that it does not possess certain organs found in the latter. The stalk is represented by a long muscular cord, which is merely a narrow part of the body. Basally the stalk of each individual passes into a common axis, which is for the most part attached to the substance on which the colony is growing, and is to some extent branched. The muscular stalk can be contracted into a spiral, thereby retracting the animal into its tube. The stalks and the younger parts of the axis which connects them are soft, but the older parts secrete a dark brown cuticle, forming a narrow tube which becomes embedded in the adherent wall of the outer tube. The thin dark axis, to which the name Rhabdopleura refers, is the feature by which the animal can most readily be recognised without magnification.
The outer transparent tube is constructed by the proboscis, or buccal shield, the secretion of which appears to be intermittent, so that the tube consists of a series of rings piled on one another. The animal crawls up the inside of its tube by means of its proboscis, while it is retracted by means of the muscles of its stalk.
The growing axis ends in a row of young buds, the buccal shields of which early reach a relatively large size. The terminal bud gives rise to tube-rings, so that the axis is surrounded by a cylindrical outer tube, which becomes interrupted by transverse septa, each bud, except the end one, thus lying in a closed chamber. The wall of each chamber becomes perforated, and the buccal shield then prolongs this perforation by adding tube-rings, the formation of which continues till the tube reaches a considerable length. The bud remains connected with the axis by means of its narrow proximal region, which forms its stalk. The adherent part of the adult colony thus consists of a row of short tubes, traversed by the common axis of the colony. Each tube is produced laterally into the upright tube of an individual.
The general anatomy closely resembles that of Cephalodiscus.[[49]] There are five body-cavities and a notochord. Collar-pores exist, but proboscis-pores and gill-slits have not been described. The dorsal region of the collar bears only a single pair of arms.
Order III. Phoronidea.
The structure and development of Phoronis (Fig. 13), have already been described in Vol. II.[[50]] of this series; and Masterman's investigations, then published in a preliminary form only, are there alluded to. Since then this author has published fuller accounts[[51]] of his results, which, if substantiated, would indicate a near relationship between Cephalodiscus and Phoronis.
Phoronis is a small tubicolous animal, of gregarious habits, which has usually been regarded as related to the Gephyrea. Its body ends in a plume of ciliated tentacles, which can be protruded from its tube, and the anus is on the dorsal side, not far from the mouth. In both these respects it agrees with Cephalodiscus, but a more striking similarity is asserted by Masterman to exist between the latter and Actinotrocha, the larval stage of Phoronis. The prae-oral ciliated hood (Fig. 14) of Actinotrocha is regarded as the proboscis, and it contains a median cavity, traversed, like that of Balanoglossus, by muscular fibres. The collar is the region between the constricted neck and an oblique line, parallel to and immediately behind the series of tentacles, which thus belong to the collar. This division has a collar-cavity which is said to be distinct from the prae-oral cavity, and is separated by a septum from the posterior body-cavity. Its dorsal epidermis contains the central nervous system (n.s), which is connected with a system of nerves resembling those of Balanoglossus. A median diverticulum of the alimentary canal of this part may be compared with the notochord of that animal, but there are no gill-slits.
Fig. 13.—Phoronis buskii, M‘Intosh, Philippine Islands, x about 2. (After M‘Intosh, from Shipley.)