Phoronis has long been regarded as a possible ally of Rhabdopleura
(see Pterobranchia); and Masterman (10) has attempted to
demonstrate the existence in Actinotrocha of most of the structures
which occur in the Pterobranchia. According to his view the
praeoral hood of Actinotrocha (cf. fig. 4) corresponds with the
“proboscis” of Pterobranchia; the succeeding region, as far as
the bases of the tentacles, with the collar; and the post-tentacular
region with the metasome. Masterman’s more detailed comparisons
have for the most part been rejected by other morphologists. One
of the most formidable difficulties in the way of the attempt to
reduce Actinotrocha to the Pterobranchiate type of structure is the
condition of the coelom in the former. There is indeed a perfectly
definite transverse septum which divides the body-cavity in the
region of the tentacle-bases. Even if it be admitted that the
postseptal space may be the metasomatic cavity, the praeseptal space
can hardly be regarded as coelomic in nature, since it is in continuity
with the vascular system; while Masterman’s conclusion that the
cavity of the praeoral hood (the supposed proboscis-cavity) is
separated from that of the supposed collar has received no
confirmation. In spite of these difficulties it must be conceded that
the dorsal flexure of the alimentary canal of the Pterobranchia
is very Phoronis-like. It has, moreover, been shown (see especially
Goodrich, 5) that shortly before its metamorphosis, Actinotrocha
develops a coelomic space which lies immediately in front of the
oblique septum, and gives rise later to the cavity of the lophophore
and tentacles. Regarding this as a collar-cavity, it becomes
possible to agree with Masterman that the region shown in fig. 4, 1.
between the tentacles and the praeoral hood, is really a collar
the coelom of which develops relatively late. It will be noticed
that the lophophore of Phoronis is, on this assumption, a derivative
of the collar just as it is in the Pterobranchia. The epistome of
the adult Phoronis cannot well be the proboscis since its cavity is
continuous with the lophophoral coelom, and because the praeoral
hood of Actinotrocha is entirely lost at the metamorphosis. It is
possible that this consideration will account for the want of an
anterior body-cavity in Phoronis. Since the proboscis is a purely
larval organ in this genus it may be supposed that the coelomic
space which properly belongs to it fails to develop, but that the
praeoral hood itself is none the less the morphological representative
of the proboscis. In spite of the criticisms which have been made
on the conclusion that Phoronis is allied to the Pterobranchia, it
is thus possible that the view is a sound one, and that the Phoronidea
should take their place, with the Enteropneusta and the
Pterobranchia, as an order of the Hemichordata.
Bibliography.—(1) Benham, Quart. Journ. Mic. Soc. xxx. 125 (1890), (2) Caldwell, Proc. Roy. Soc. xxxiv. 371 (1883); (3) Cori, Zeitschr wiss. Zool. li. 480 (1891); (4) Fowler, art. “Hemichorda,” Ency. Brit. xxix. 249 (1902); (5) Goodrich, Quart. Journ. Mic. Soc. xlvii. 103 (1904); (6) Harmer, Siboga Rep. xxvi. 114, bis (Pterobranchia), (1905); (7) Ikeda, J. Coll. Sci. Japan, xiii. 507 (1901); (8) Lankester, art. “Polyzoa,” Ency. Brit. xix. 430, 433 (1885); (9) De Selys-Longchamps, Arch. Biol. xviii. 495 (1902); Wiss. Meeresunt. (N. F.) vi. Abt Helgoland (1903), Heft i.; Mém classe sci. acad. belgique, vol i. (1904); Fauna u. Flora G. v. Neapet, 30 Monogr. (1907); (10) Masterman, Quart Journ. Mic. Soc. xl. 281 (1898); xliii. 375 (1900); (11) Schultz, Zeitschr. wiss. Zool. lxxv. 391, 473 (1903); (12) Shearer, Mitth. zool. Stat. Neapel, xvii. 487 (1906); 13) Shipley, Cambr. Nat. Hist. ii. 450 (1896). (S. F. H.)
PHORORHACOS, the best-known genus of the extinct
Patagonian Stereornithes (see Bird: Fossil). Among the bones found in the strata of the Santa Cruz formation (now considered
as mainly of mid-Miocene date) was the piece of a mandible
which F. Ameghino described in 1887 as that of an edentate
mammal, under the name of Phorysrhacos longissimus (Bolet.
Mus. de la Plata, i. 24). In 1891 (Rev. Argent. Hist. Nat. i. 225)
he amended the name and recognized the bone as that of
a bird, Phororhacos, which with Brontornis and others constituted
the family Phororhacidae. About six species of the
type genus are now known, the most complete being Ph. inflatus,
with skull, mandible, pelvis, limbs and some of the vertebrae.
These birds were at first considered as either belonging to the
Ratitae, or at least related to them, until C. W. Andrews, after
much of the interesting material had been acquired by the British
Museum, showed the gruiform affinities of Phororhacos (Ibis,
1896, pp. 1–12), a conclusion which he was able to further corroborate
after the clearing of the adherent stony matrix from the
skulls (Tr. Z. S. 1901, xv. pp. 55–86, pls. 14–17). The skull
of Ph. longissimus is about 2 ft. long and 10 in. high; that
of Ph. inflatus is 13 in. long, and this creature is supposed
to have stood only 3 ft. high at the middle of the back. The
under jaw is slightly curved upwards and it contains a large
foramen as for instance in Psophia and in Mycteria. The
strongly hooked upper beak is very high, and very much compressed
laterally. The palate is imperfectly desmognathous,
as in Dicholophus, with an inconspicuous vomer. The quadrate
has a double knob for its articulation with the skull, and
basipterygoid processes are absent. What l1ttle is known of the
shoulder-girdle (breastbone still unknown) points to a flightless
bird, and so do the short wing bones, although these are stout.
The pelvis has an ischiadic foramen. The hind limbs are
distinctly slender, the tibia of Ph. inflatus being between 15 and
16 in. in length.
|
| (From life-size model in Brit. Mus. Nat. Hist.) |
| Skull of Phororhacos, longissimus. |
For further detail see F. Ameghino, “Sur les oiseaux fossiles de la Patagonie,” Bolet. inst. geogr. argentino, xv., chs. 11 and 12 (1895); F. P. Moreno and A. Mercerat, Catálogo de los pájaros fósiles de la República Argentina, An. Mus. La Plata (1891; with 21 plates). (H. F. G.)
PHOSGENITE, a rare mineral consisting of lead chlorocarbonate,
(PbCl)2CO3. The tetragonal (holosymmetric) crystals
are prismatic or tabular in habit, and are bounded by smooth,
bright faces: they are usually colourless and transparent, and
have a brilliant adamantine lustre. Sometimes the crystals
have a curious helical twist about the tetrad or principal axis.
The hardness is 3 and the specific gravity 6.3. The mineral is
rather sectile, and consequently was early known as “corneous
lead” (Ger. Hornblei). The fanciful name phosgenite was
given by A. Breithaupt in 1820, from phosgene, the old name
of carbon oxychloride, because the mineral contains the elements
carbon, oxygen and chlorine. At Cromford, near Matlock, it
was long ago found in an old lead mine, being associated with
anglesite and matlockite (Pb2OCl2) in cavities in decomposed
galena: hence its common name cronfortite. Fine crystals are also
found in galena at Monteponi near Iglesias in Sardinia, but the
largest are those recently found near Dundas in Tasmania.
Crystals of phosgenite, and also of the corresponding bromine
compound [PbBr]2CO3, have been prepared artificially.
(L. J. S.)
PHOSPHATES, in chemistry, the name given to salts of
phosphoric acid. As stated under Phosphorus, phosphoric
oxide, P2O5, combines with water in three proportions to form
H2O·P2O5 or HPO3, metaphosphoric acid; 2H2O·P2O5 or H4P2O7,
pyrophosphoric acid; and 3H2O·P2O5 or H3PO4, orthophosphoric
or ordinary phosphoric acid. These acids each give origin to
several series of salts, those of ordinary phosphoric acid being
the most important, and, in addition, are widely distributed
in the mineral kingdom (see below under Mineral Phosphates).
Orthophosphoric acid, H3PO4, a tribasic acid, is obtained by boiling a solution of the pentoxide in water; by oxidizing red phosphorus with nitric acid, or yellow phosphorus under the surface of water by bromine or iodine, and also by decomposing a mineral phosphate with sulphuric acid. It usually forms a thin syrup which on concentration in a vacuum over sulphuric acid deposits hard, transparent, rhombic prisms which melt at 41.7°. On long heating the syrup is partially converted into pyrophosphoric and metaphosphoric acids, but on adding water and boiling the ortho-acid is re-formed. It gives origin to three classes of salts: M′H2PO4 or M″H4P2O8; M′2HPO4 or M″HPO4, M′3PO4, M″3P2O8 or M‴PO4, wherein M′, M″, M‴ denote a mono, di- and tri-valent metal. The first set may be called monometallic, the second bimetallic, and the third trimetallic salts. Per-acid salts of the alkalis, e.g. (K, Na, NH4)H5(PO4)2, are also known; these may be regarded as composed of a monometallic phosphate
