Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/455

P L Y Z A 435 surface by which the tube or case (tubarium) in which the polypide is enclosed is secreted. Both Rhabdopleura and Cephalodiscus produce colonies by budding ; but the colonies of the former are large, definite, and arborescent, whilst those of Cephalodiscus are remarkable for the fact that the buds do not remain long in organic continuity with their parent, but become detached and nevertheless continue to be enclosed by the same common envelope or secretion. The bud-formation of Ilhabdopleura recalls that of Paludicella in the fact that it leads to the formation of continuous arboriform com munities. That of Cephalodiscus resembles the budding of Loxosoma, since no two fully-formed individuals remain II in organic continuity. Both Rhabdopleura and Cephalo discus (like Phoronis) produce cases or investments in which they dwell. These are free secretions of the organ ism, and are not, like the ccenoecia of Eupolyzoa, cuticular structures adherent to and part of the polypide s integu ment. The dwelling of Rhabdopleura is a branched system of annulated tubes of a delicate membranous con sistency, each tube corresponding to a single polypide, the rings of which it is built being successively produced at the termination of the tube by the secreting activity of the prae-oral disk (fig. 7, E). The polypides freely ascend and descend in these tubes owing to the contractility of their stalks. On the other hand the dwelling of Cephalodiscus , is a gelatinous, irregularly branched, and fimbriated mass (fig. 8), excavated by numerous cavities which communicate with the exterior. In these cavities are found the nu merous detached small colonies of Cephalodiscus (fig. 9), or we should rather say the isolated budding polypides. The remaining important feature in the organization of the Ptero- branchia, namely, the parts connected with the forma tion of buds, are best un derstood by first examining Cephalodiscus. The body of Cephalodiscus is seen (fig. 9) to be an oval sac ; in this is suspended the U-shaped alimentary canal, and from the walls of its cavity (coelom) the ova and the spermatozoa are de veloped. Projecting from the ventral face of this oval sac is a muscular cy lindrical stalk, into which the viscera do not pass, though the ccelom is con tinued into it (fig. 9, c). This stalk is merely the outdrawn termination of the body. long as the whole of the rest of the animal, and it is from its extremity that the buds are produced (fig. 9, a). Before the buds have attained half the size of their parent they become detached, but continue to occupy some portion of the common gelatinous dwelling. Tt i alirmf an FIG. 8. Dwelling of gelatinous consistence and brown colour formed by the polypides of Cephalodiscus doilecalofihiif, M Intosh; natural size (from an original drawing kindly supplied by Prof. M-Intosh, F.K.S.). o. polypide within the jelly ; p, cavity once occupied by polypides. FIG. 9. A polypide of Cephalvtliscus dculecatophus removed from the gelatinous house (from an original drawing by Prof. M Intosh). >^"o organic connexion has been severed in thus isolating this polypide with its attached buds a, o. The figure represents the furthest point to which colony-formation attains in this form, a, buds growing from the bas of the polypide-stalk : b, the prae- oral lobe (buccal shield or disk); f, the polypide-stalk; rf, the ciliated tentacles of the twelve lophophore arms (six pairs, each like the single pair of Rhab dopleura) inextricably matted and confused ; e, anterior margin of the prae- oral lobe ; /, p sterior margin of the same. Magnified about fifty times linear Turning to Rhabdopleura, we find that each polypide has a body of similar shape and character to that described for Cephalodiscus, and a similar ventrally developed

" stalk :; (fig. 7, A, <). But, inasmuch as the buds deve-