Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/449

Rh Anthozoa and Bryozoa. From 1830 onwards there was a distinct tendency to separate the Polyzoa from the polypes they are spoken of as higher Polypi, compound polypes, &c. In 1831 Ehrenberg described the Polypi or Curialia as a class of the Phytozoa, and distinguished the true polypes or Anthozoa from the Bryozoa. Milne-Ed wards expressly excludes the Bryozoa, and restricts the term, polype to the Zoantharia and Alcyonaria inclusive of Hydra and Lucer- naria. In 1847 Frey and Leuckart placed the Polyzoa by themselves, and united the remaining Polypi of Cuvier along with the Acalephx under the class name Ccelenterata, which the latter afterwards (1853) divided into (1) Cteno- p/tora, (2) Medusx, and (3) Polypes. Modern anatomists generally agree in confining the term to the individuals (zooids or personal) of hydriform Hydromedusse, and Actinozoa, and frequently restrict it to the former (see HYDROZOA, CORALS).  POLYPTERUS, a genus of Ganoid fishes common in many rivers of tropical Africa, and known on the Nile by the name of abu bishir. Their body is cylindrical in shape, elongate, and covered with hard, polished, ganoid scales, which are arranged in oblique series. The head, with flattened snout and wide mouth, is protected by bony plates with ganoid external surface, of which a series of &quot;supra-temporal&quot; and &quot;spiracular&quot; ossicles are especially characteristic. Spiracles, or external openings of a canal leading into the pharynx are persistent throughout life, .situated on each side of the parietal bone, and closed by an osseous valve. The lips are fleshy, but the space between the rami of the mandible is covered by a large &quot;gular&quot; plate. The vent is placed far backwards, in front of the anal fin, the tail being short, with a diphy- cercal termination of the vertebral column. The mouth is well provided with rasp-like teeth, forming broad bands in the jaws, on the vomer and palatine bones. The paired fins are supported by an axial skeleton. The structure of the dorsal fin is unique : its anterior portion is composed of isolated finlets, from eight to eighteen in number, each of which consists of a flattened spine with a bifurcate termination ; to the posterior aspect of the top of each spine several soft rays are attached, which result from the dichotomous division of a single ray, the basal portion of which is the spine. Posteriorly these finlets pass into the ordinary rays composing the caudal fin, which surrounds the tail. The ventral fins are well- developed, and inserted behind the middle of the length of the trunk. The respiratory apparatus consists of three and a half gills, and is protected by an osseous gill-cover. An external gill of considerable size in the form of a tapering band fringed with respiratory lamina? exists in young examples, and is attached to the end of the gill- cover. The air-bladder is double, and communicates with the ventral wall of the pharynx. Such, are some of the principal characteristics of one of the most interesting representatives of a type which in Polyptcrus lias survived from the Devonian and C arlioniferous formations to our period ; for further details of its internal organization sec ICHTHYO LOGY. The centre of distribution of Pvhjptcrus is the lake region of tropical Africa, from which the Nile and the great rners of West Africa take their origin. A very remarkable fact is ils total absence in the East-African river systems which belong to tl.e Indian Ocean. Specimens of the bishir have been found in the Nile as low as Cairo, but it is very scarce throughout the middle and lower parts of that river ; such individuals have evidently been carried by the current down from southern latitudes, nnd do not propagate the species in the northern parts. As mentioned above, the number of the rays which are modified into finlets varies considerably, and consequently several species have been distin guished by some naturalists, whilst others hold that the&amp;gt;c is one species of I vJyptcnis only. The largest specimens observed had a length of 4 feet. Nothing is known of its habits and propagation, and observations thereon are very desirable. Some years ago an extremely interesting dwarf form of Polyptcrus was discovered in Old Calabar, and described under the name of Calainoichlhys caldbaricus. It much resembles the bishir but is smaller, and considerably more elongate. POLYPUS, a term in surgery, signifying a tumour which is attached by a narrow neck to the walls of a cavity lined with mucous membrane. A polypus or polypoid tumour may belong to any variety of tumour, either simple or malignant. The most common variety is a polypus of the nose of simple character and easily removed. Polypi are also met with in the ear, larynx, uterus, vagina, and rectum. See.  POLYSPERCHON, one of Alexander's generals, and the successor of Antipater as regent in Macedonia in 318 B.C. He was driven from the kingdom by Cassander in 316. For the leading incidents of his brief term of office see PHOCION (vol. xviii. p. 800) ; compare also MACEDONIA.  POLYXENA, in Greek legend, a daughter of Priam, last king of Troy, and Hecuba. She had been betrothed to Achilles, and after his death and the destruction of Troy the ghost of Achilles appeared to the returning Greeks as they were encamped on the Thracian Chersonese and demanded of them the sacrifice of Polyxena. The Greeks consented and Neoptolemus, son of Achilles, sacrificed Polyxena on his father's grave. This tragic story is the subject of the Hecuba of Euripides and the Troades of Seneca. Of Sophocles's tragedy Polyxena a few fragments only remain.  POLYZOA is the name applied by J. Vaughan Thompson in 1830 (1) to a group of minute polyp-like organisms which were subsequently (1834) termed &quot;Bryozoa&quot; by Ehrenberg (2). The forms included in this group were stated by Thompson to be “in a general way the whole of the Flustracete, in many of which I have clearly ascertained the animals to be Polyzoæ,” they having been previously considered by zoologists to be allied to the Hydra-like polyps. These organisms had previously been known by the hard corneous &quot; cells &quot; or chambers which are formed by the animals on the surface of their bodies, and build up, in consequence of the formation of dense colonies by bud- ding, complex aggregates known as &quot; sea mats &quot; and &quot; sea mosses.&quot; Thompson expressly stated the opinion that the organization of the animals detected by him led to the conclusion that &quot; they must be considered as a new type of the Mollusca Acephala.&quot;

Subsequently (1844) Henri Milne-Edwards (3) pointed out the relationship of Thompson s Polyzoa to the Brachiopoda, and, adopting the latter s view as to their Molluscan affinities, proposed to unite these two classes with the Tunicata in a group to be called &quot; Molluscoidea.&quot; Re cent researches have entirely separated the Tunicata from this association, and have demonstrated that they belong to the great phylum of Vertebrata. On the other hand the association of the Polyzoa with the Brachiopoda appears at present to be confirmed, though the relationship of these two classes to the Mollusca has been shown to rest on mistaken identification of parts; see, however, Harmer (18).

The Polyzoa appear to be related to the Sipunculoid Gephyrrean worms (Gephyræa inermia) more nearly than to any other class of the animal kingdom. The study and interpretation of the facts of their ontogeny (growth from the egg) presents such extreme difficulty that in the pre sent state of our knowledge it is necessary to regard them 