Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/456

436 P L Y Z A loped on the stalk of a Rhabdopleura polypide do not detach themselves, we find that we can trace the stalk of each polypide of a colony into connexion with the stalk of the polypide from which it was originally budded, which may now be considered as a " branch " bearing many- stalked polypides upon its greatly extended length, and such a " branch-stalk " may be further traced to its junc tion with the " stem-stalk " of the whole colony. The stem-stalk was at one time the simple terminal stalk of a single polypide, but by lateral budding it gave rise to other polypides, and so became a gemmiferous "branch"; and further, when some of these in their turn budded and became branches, it became the main "stem" of a copious colony. A serious error has been made in comparing the contrac tile stalk of the Pterobranchiate polypide to the " f uni- culus or cord-like mesentery of Eupo- lyzoa. With this it has morphologi cally nothing in common, since it is not an internal organ, but simply the elongated termination or stalk of the body, comparable to the stalk of Pedi- cellina (fig. 15) and Loxosomi (fig. 16), or to the hydrocaulus of such a Hydro- zoon colony as Cordylophora. The stalk where it bears only very young buds, or none at all, 43 is always its condition in Cephalodiscus and in many polypides of a Rhabdopleura colony, miy bs called a "gymnocaulus"; when once its buds have devel oped into full grown poly pides, and it has elongated proportionally with their growth, it becomes a " pec- FlG . 10 ._ A polypide of Cepha odiscus do- tocaulus"; that is to say, it is to that part of it which bears such polypides that this term may be conveni ently applied. The pecto- caulus of Rhabdopleura, both in the form of branch and stem, undergoes remarkable change of appearance as com pared with the gymnocaulus. It loses its contractility, shrinks, and develops on its surface a hard, dark, horny cuticle (whence its name), comparable precisely in its nature to the hardened cuticle which forms the zocecia of Eupo- lyzoa. It now has the appearance of a black cord or rod-lik^ body lying within and adherent to the inner face of thj much wider tubular stem, and branches formed by the gradual building up and arborescent extension of the annulated tubarium secreted by the individual polypides. It has been regarded both by Allman and by Sars as a special structure, and called by the former " the chitinous rod" or " blastophore," by the latter "the axial cord." In reality it is the black-coloured pectocaulus Rhabdopleura which corresponds to the ccencecium of an ordinary Polyzoon ; whilst the term " ccenoecium " is totally inapplicable morphologically to the annulated branched tube in which the Rhabdopleura colony lives, this having absolutely no parallel in the Eupolyzoa. A sac-like testis has been discovered in Rhabdopleura opening by the side of the anus (Lankester, 7) ; but the ova have not yet been seen, nor is anything known of its development. Similarly the eggs of Cephalodiscus are observed within the body of the parent in the "Chal lenger" specimens, but nothing further is known of its life-history. A body-cavity is present (Lankester), though its exist ence has been denied by Sars and by M Intosh. Neph- ridia and nerve ganglia are not described. Cephalodiscus decalophus, from which the lophophore- tentacles and buccal shield have been removed in order to show the remark able eyes, a, buils ; c, stalk; <i, eyes; h, post-oral collar, hi lden by the buccal shield i i fig. 9. (Original drawing by Prof. M Intosh F.R.S.) Of has two remarkable eye spots dorsal to the cephalic disk (ng- 10, y). THE EUPOLYZOA. Whilst it is necessary to include in the group Polyzoa the forms we have already noticed as Yermiformia and Pterobranchia, there can be no doubt that those organisms to which we assign the name Eupolyzoa are primarily those upon which naturalists have framed their concep tion of the group, and that they constitute a very con sistent assemblage, held together by well-defined characters, and yet presenting an immense number of varied forms showing a wide range of modifications. All the Eupolyzoa have closely approximated mouth and anus, and, like Paludicella, a complete range of hollow ciliate tentacles, describing either a circle or a horse shoe, surrounding the mouth. The anus as well as the mouth is included in this area in a few exceptional forms (the Entoprocta); it lies near but outside the lophophore (as the area is termed) in the vast majority (the Ectoprocta). Except in the Entoprocta, where the movement is limited, the whole anterior portion of the body bearing the lophophore can be invaginated into the hinder part (as described above for the typical Eupolyzoon Paludicella). This character distinguishes the Eupolyzoa from both Vermiformia and Pterobranchia. The polypides of all the Eupolyzoa are minute, but all produce buds which remain in organic continuity with their parent (except in Loxo- soma) and build up very considerable and sometimes massive colonies. In all Eupolyzoa the cuticle of the hinder part of each polypide is thick and dense, thus forming a hard -walled sac, the zocecium. This is peculiar to and universal in the Eupolyzoa (except Loxosoma), and is not to be confounded with the non-adherent tubes of Phoronis and Rhabdopleura or the jelly-house of Cephalodiscus. The connected zocecia of a colony of Eupolyzoa constitute a ccenoecium. A simple nerve ganglion between mouth and anus, a large body-cavity (except in Entoprocta), simple gonads without accessory glands or ducts, usually testis and ovary in the same polypide, absence of a blood-vascular system, of any but the most rudimentary nephridia, and of eyes, otocysts, or other special sense-organs, are features characterizing all adult Eupolyzoa. The section Eupolyzoa, with its vast number of species and genera, requires a somewhat elaborate classification. The forms in which the anus is enclosed within the tentacular circle are very few, and are peculiar in other respects. We follow Nitschc (8) in separating them as the sub-class Entoprocta from the majority of Eupolyzoa forming the sub-class Ectoprocta. Sub-class 1. Ectoprocta, Nitsche. Eupolyzoa with the anus not included within the area of the lophophore. Anterior portion of the body of the normal polypide introversible. Tentacles not individually capable of being coiled or flexed. Order 1. PHYLA.CTOLVEMA, Allman. Ectoproctous Eupolyzoa in which the polypide possesses a prae-oral lobe or epistome, similar to that of Phoronis, and comparable to the more highly developed buccal shield or disk of the Pterobranchia. Lophophore (except in Fredericella, where it is nearly circular) horse-shoe-shaped (hippocrepian). Polypides of a colony equi-formal, that is, not differentiated in structure and function. Neighbouring zoujcia are in free and open communication, the bud never becoming shut off by a perforated cuticular plate from its parent. Cuticle of the zocecia either gelatinous or horny,

forming massive or else arborescent ccencecia, in one genus