Page:EB1911 - Volume 22.djvu/56

 
 * (J. G. F.; W. G.)

 POLYXENA, in Greek legend, daughter of Priam, king of Troy, and Hecuba. She had been betrothed to Achilles, who was slain by Paris in the temple of Apollo Thymbraeus, where the marriage was to have been celebrated (Hyginus, Fab. 110). The shade of Achilles afterwards appeared to the returning Greeks in the Thracian Chersonese and demanded the sacrifice of Polyxena, who was put to death by Neoptolemus, son of Achilles, on his father’s grave (Ovid, Metam. xiii. 440 sqq.). The tragic story is the subject of the Hecuba of Euripides, the Troades of Seneca and the Polyxena of Sophocles, of which only a few fragments remain. According to Philostratus (Heroica, 20, 18), Polyxena fled to the Greeks after the murder of Achilles and committed suicide on his tomb.  POLYZOA, in zoology, a term (introduced by J. V. Thompson, 1830) synonymous with Bryozoa (Ehrenberg, 1831) for a group commonly included with the Brachiopoda in the Molluscoidea (Milne Edwards, 1843). The correctness of this association is questionable, and the Polyzoa are here treated as a primary division or phylum of the animal kingdom. They may be defined as aquatic animals, forming colonies by budding; with ciliated retractile tentacles and a U-shaped alimentary canal. The phylum is subdivided as follows.

Class I. (Nitsche). Lophophore circular, including both mouth and anus. Tentacles infolded, during retraction, into a vestibule which can be closed by a sphincter. Body-wall not calcified, body-cavity absent. Definite excretory organs present. Reproductive organs with ducts leading to the vestibule. Zooids possessing a high degree of individuality. Loxosoma, Pedicellina (fig. 1), Urnatella.

Class II. (Nitsche). Lophophore circular or horseshoe shaped, including the mouth but not the anus. Tentacles retractile into an introvert (“tentacle-sheath”). Body-wall membranous or calcified, body-cavity distinct. Specific excretory organs absent, with the doubtful exception of the Phylactolaemata. Reproductive organs not continuous with ducts. Zooids usually connected laterally with their neighbours.

Order 1. (Allman).—Lophophore circular, with no epistome. Body-cavities of zooids not continuous with one another. Body-wall not muscular.

Sub-order 1. (Ulrich); Fossil.—Zooecia, long and coherent, prismatic or cylindrical, with terminal orifices, their wall thin and simple in structure proximally, thickened and complicated distally. Cavity of the zooecium subdivided by transverse diaphragms, most numerous in the distal portion. Orifices of the zooecia often separated by pores (mesopores).

Sub-order 2. (Vine); Fossil.—Zooecia usually short. Orifice concealed at the bottom of a vestibular shaft, surrounded by a solid or vesicular calcareous deposit.

Sub-order 3. (Busk).—Zooecia prismatic or cylindrical, with terminal, typically circular orifice, not protected by any special organ. The ovicells are modified zooecia, and contain numerous embryos which in the cases so far investigated arise by fission of a primary embryo developed from an egg. Crisia (fig. 2), Tubulipora, Hornera, Lichenopora.

Sub-order 4. (Busk).—Zooecia with soft uncalcified walls, the external part of the introvert being closed during retraction by a membranous collar. Zooecia either arising from a stolon, without lateral connexion with one another, or laterally united to form sheets. Alcyonidium, Flustrella, Bowerbankia (fig. 3), Farrella, Victorella, Paludicella.

Sub-order 5. (Busk).—Zooecia with more or less calcified walls. Orifice closed by a lid-like operculum. Polymorphism usually occurs, certain individuals having the form of avicularia or vibracula. The ovicells commonly found as globular swellings surmounting the orifices are not direct modifications of zooecia, and each typically contains a single egg or embryo. Membranipora, Flustra, Onychocella, Lunulites, Steganoporella, Scrupocellaria, Menipea, Caberea, Bicellaria, Bugula, Beania, Membraniporella, Cribrilina, Cellaria, Micropora, Selenaria, Umbonula (fig. 4), Lepralia, Schizoporella, Cellepora, Mucronella, Smittia, Retepora, Catenicella, Microporella, Adeona.

Order 2. (Allman).—Lophophore horse-shoe shaped, or in Fredericella circular. Mouth guarded by an epistome. Body-cavities of zooids continuous with one another. Body-wall uncalcified and muscular. Reproduction sexual and by means of “statoblasts,” peculiar internal buds protected by a chitinous shell. Fredericella, Plumatella (fig. 5), Lophopus, Cristatella, Pectinatella.

Hatschek (1888) treated the Entoprocta as a division of his group Scolecida, characterized by the possession of a primary body-cavity and of protonephridia; while he placed the Ectoprocta, with the Phoronida and Brachiopoda, in a distinct group, the Tentaculata. Against this view may be urged the essential similarity between the processes of budding in Entoprocta and Ectoprocta (cf. Seeliger, Zeitschr. wiss. Zool. xlix. 168; l., 560), and the resemblances in the development of the two classes.

Of the forms above indicated there is no palaeontological evidence with regard to the Entoprocta. The Trepostomata are in the main Palaeozoic, although Heteropora, of which recent species exist, is placed by Gregory in this division. The Cryptostomata are also Palaeozoic, and include the abundant and widely-distributed genus Fenestella. The Cyclostomata are numerous in Palaeozoic rocks, but attained a specially predominant position, in the Cretaceous strata, where they are represented by a profusion of genera and species; while they still survive in considerable numbers at the present day. The Ctenostomata are ill adapted for preservation as fossils, though remains referred to this group have been