This name has recently-been proposed by Mr Moseley for two groups of marine animals which produce a regular skeleton of carbonate of lime, often of large size, and which have been generally referred to the Corals (Adinozoa). One of these groups comprises the well-known Millepora (fig. 64), which is found contributing so largely to the formation of coral-reefs in the West Indies and Pacific. The calcareous skeleton of Millepora is mostly in the form of foliaceous or laminar expansions, stony in texture, with a smooth surface studded with minute apertures of two sizes, the larger of these being much the fewest (fig. 64, C). The larger openings are the mouths of tubes (fig. 64, B, p p), which are divided by transverse calcareous partitions into a number of compait-ments, only the most superficial of these being actually tenanted by the living animal. The smaller tubes are similarly septate or "tabulate," and the general tissue of the skeleton (fig. 64, C) is composed of calcareous trabeculae traversed by a series of ramifying and anastomosing coe-nosarcal canals, which place the tabes occupied by the zooids in direct communication.
Fig. 62. - A, Graptolites (Mon-ograptus) priodon, Bronn, preserved in relief - lateral view, slightly enlarged ; B, Dorsal view of a fragment of the same species - considerably enlarged; C, Front view of a fragment of the same, showing the mouths of the cellules - much enlarged ; D, Transverse section of the same. All from the base of the Coniston Flags. (Original.)
From the presence of transverse partitions, or "tabulae," in its tubes, Millepora was generally placed amongst the so-called "Tabulate Corals," with the typical forms of which it has no affinity. Though its skeleton is abundantly obtained in the regions where it occurs, the living animal has been rarely observed. The late Professor Agassiz was the first to examine Millepora in its living condition, and he was led to the conclusion that the genus was unequivocally referable to the Hydro-zoa. A similar conclusion has recently been reached by Mr Moseley, who had the opportunity of examining the living animal minutely. According to this observer, the colony (fig. 65) of Millepora consists of two kinds of zooids. The larger zooids, or "gastrozooids," inhabit the larger tubes of the skeleton, and possess from four to six knobbed tentacles; while the smaller zooids, or "dactylozooids," inhabit the smaller tubes, and are either indiscriminately mixed with the gastrozooids, or surround these in definite systems (fig. 65, a and b). The dactylozooids have no mouth, and are long and slender, carrying on their sides numerous short clavate tentacles. They perform the functions of prehension for the colony, and supply food to the stomach-bearing gastrozooids, by which the work of digestion and assimilation is carried on. The nutritive fluid elaborated by the latter is distributed throughout the entire colony by means of branched coenosarcal canals, which ramify in every direction through the spongy tissue of the skeleton. The reproductive process in Millepora is still unknown.
Fig. 63. - Diplograptus pristis, a diprio-nidian Graptolite. (Original.)
Fig. 64. - A, Portion of a mass of Millepora alcicomis, of the natural size; B, Portion of the same, cut open vertically to show the larger tabulate tubes (p p), and the spongy coenosarcal skeleton (c c), enlarged ; C, Small portion of the surface, enlarged to show the larger and smaller openings (p' and c') inhabited by the different zooids, and the reticulated calcareous tissue of the skeleton; D, One of the tentacular poly-pites, enlarged, showing two whorls of knobbed tentacles. (A, B, and C are after Milne-Edwards and Haime; D is after Martin Duncan and Major-General Nelson.)
Still more remarkable than the Milleporae are the singular organisms forming the family of the Stylasteridae, which have hitherto been regarded as Corals, but which have been shown by Mr Moseley to belong to the Hydroeorallinae. The skeleton of the Styleisteridae (fig. 66) is calcareous, more or less branched, forming a dendroid or flabellate expansion, and exhibiting upon the surface, or on its sides, small rounded apertures, which are usually intersected marginally by radiating partitions or "septa," and thus simulate the "calices" of an ordinary sclerodermic coral. In other cases, the skeleton shows a series of large apertures, with more numerous and irregularly distributed smaller openings, the latter not being radially arranged round the former. In any case, the skeleton is traversed in all directions by a system of branched and anastomosing canals, which are occupied in the living condition by prolongations of the coenosarc, which also forms an ectodermal covering to the skeleton. The colony is composed of two different sets of zooids - the one set ("gastrozooids") provided with a mouth and stomach-sac; while the others ("dactylozooids") are elongated and destitute of a mouth, thus coming to represent tentacles in form. The gastrozooids occupy, as in Millepora, the large tubes of the skeleton, and the dactylozooids, are lodged in the small tubes. Hence, when the dactylozooids are arranged in definite "cyclo-systems" round the gastrozooids, then each of the large apertures in the skeleton comes to be surrounded by a circle of smaller elongated pores, which are only separated laterally by thin partitions, and which thus give rise to the appearance of a central "calice" surrounded by radiated "septa" (fig. 66, B). The gastrozooids are not only larger than the dactylozooids, but they have a special layer of digestive cells lining the' body-cavity, a structure which is wanting in the purely prehensile dactylozooids. The true Hydrozoal character of these extraordinary organisms is conclusively shown by the fact that the reproductive organs are situated outside the bodies of the ordinary zooids, being in the form of fixed sporosacs developed within sac-like cavities ("ampullae") in the skeleton (fig. 66, B), which at certain periods communicate with the exterior by minute pores.
Fig. 65. - Enlarged view of a portion of the surface of a living colony of Millepora nodosa, showing the expanded zo5ids of a single system : a Central "gastrozooid;" b One of the mouthless " dactylozooids." (After Moseley.)
Fig. 66. - A, Portion of the skeleton of Stylaster sanguinetis, of the natural size; B, Small portion of a branch of the same, enlarged, showing the calices and ampullae. Living in the Australian Seas. (After Milne-Edwards and Haime.)