BRYOZOA.
Char.—Tentacles of the polype hollow, with ciliated margins ; alimentary canal with stomach, intestine, and anus ; polypary, when present, external, horny, and calcareous.
The metamorphoses which the Bryozoa undergo are like those of the lower Polypi ; the embryo developed from the ovum is an oval, discoid, or subdepressed body, with a general or partial ciliated surface, by which it enjoys a brief locomo tive life after its liberation from the parent. The Bryozoa are allied to the compound Asoiclia ; but not one of the ascidian Molluscoids quits the ovum as a gemmule swimming by means of cilia ; and no Bryozoon quits the ovum in the guise of a Cercarian or tadpole, to swim abroad by the alternate inflexions of a caudal appendage. In a progressive and con tinuous series of teachings, by pen or word of mouth, the place of an osculent or transitional group is governed by convenience, by considerations of how best to teach by com parison and easy gradation. The real merits of the man who would make scientific capital by changing the position of such group, and by imputing error or ignorance to the author from whom he may differ in this respect, are easily weighed and soon understood.
The Bryozoa, whether regarded as the highest organized Polypes, or as the lowest organized Mohasco, or as an inter mediate type, are treated of in systematic palaeontology in the position here assigned to them. The practical palaeontologist finds himself compelled to arrange and study the fossil Bryozoa along with the corals, if only on account of the diffi culty he, in many cases, experiences of determining to which class of Polypi his specimens belong. M. d'Orbigny, who has devoted much attention to this class, enumerates 544 fossil species, distributed in 73 genera. This number must be very far below the real one, since the Bryozoa of the chalk, which alone have been carefully examined, amount to 213 species ; while only two species are known from the trias, none at all from the liar, and only five from the upper oolites, so rich in corals and sponges. In the " Cours Elementaire" of d'Orbigny
the fossil Bryozoa are stated to amount to 1676.
Of the 19 or 20 palwozoic genera, none extend into the secondary strata ; but of the 18 oolitic genera, Entcdophora and Defrancia range onwards to the tertiaries ; and Alecto, Idmenea, and Eschara still survive. The oldest known fossil, Oldhantia (fig. 3, 2), is supposed to be a Bryozoon. The most common palozoic form is Fenestrella (fig. 3, i 1), resembling the recent " lace-coral"; there are 35 species, ranging from the Lower Silurian to the Permian. One of its modifications re sembles a feather (Ptilopora, fig. 3, 10, and is found in the carboniferous limestone. Another, more remarkable, has a spiral axis (Arch,intedipora, fig. 3, 9), and occurs in the same formation in Kentucky. One of the oldest genera is Ptilo dictya (fig. 3, s), of which seven species are found in the Lower Silurian formations. The slabs of Silurian limestone obtained at Dudley are covered with myriads of small and delicate fossils, including many Bryozoa. Some of these are spread like a film over other fossils, and have been doubtfully re ferred to the modern genera Discopora and Berenicea ; others, with slender branches, and erect or creeping, are called Mille poras, Heteroporas, and Escharinas. The genus Granites (fig. 3, 7), perhaps belongs here. The magnesian limestone con tains several large " lace-corals" of the genera Fenestrella, Synocladia, and Phyllophora ; and two branching species of Thamniscus and Acanth,oe,ladia. The oolites afford many small incrusting species related to Diastopora, and branching forms like Terebellaria and Ch,rysaora. In the chalk, the Escharas are most numerous, and Lunulites and Cupularkt first appear. Some thin beds of the lower chalk are almost composed of Bryozoa, mingled with Foraminifera. The coralline crag of Suffolk takes its name from the great abundance of Bryozoa it contains, among which Eschara, Cdlepora, Fascicularia, Theo noa, Hornera, Idmonea, Flustra, and Tubulipora are the most important