402 



Poi.YZOiV.- 



-RADIATA.- 



-HlPPOCKKPIA. 



that tliioiigUout these vast areas, wherever there is now 

 an " Atoll," a foundation must have originally existeil 

 within a depth of from twenty to thirty fathoms from 

 the surface." Taking this as a certainty, he solves all 

 difficulty as to the origin of these immense structures, 

 by saying that wherever these " Atolls" occur there 

 must have been a gradual subsidence of land, which 

 served as a basis for the polypiferous corals to erect 

 their wonderful structures upon. The Polypes never 

 build their corals above low- water mark ; hence the 

 coral islands and reefs never rise many feet above the 

 sm-face. The elevation of the land upon them is prin- 

 cipally owing to the action of the sea. Large masses 

 are detached by the waves and washed upon the reef; 

 shells, coral, and other debris are likewise waslied up 

 on to it, and accumulate till a surface is formed with a 

 l:iyer of soil upon it capable of supporting various forms 

 of vegetable life. 



There are numerous genera and species of coral, the 

 variations of form depending upon the manner in which 

 the animal emits from the whole surface, or from a 

 particular part of the sides of its body, the bud by 

 which the new individuals of the general mass or society 

 are produced. The specimens usually shown in col- 

 lections are small individuals which grow in sheltered 

 ]ilaces among the rocks, where they are not exposed 

 to the action of the waves, and collected before they 

 have reached their proper magnitude. The illustra- 

 tions in Plate 2 of the various genera of corals, give 

 a very poor idea of the beauty of these formations. 

 In the family MadrephylliidiB the genus CarynphyUia 

 is represented on Plate 2, figs. 6-7, by the Caryopliyllia 

 solUaria. The genus Astroea is represented at figs. 

 4-5, by Astra:a ananas; the genus Meandrina, or Brain 

 Coral, at figs. 10-11 by Meandrina lirnosa; and at fig. 3 

 by M. cerchrifurmis ; and the genus Oculiiia is repre- 

 sented at figs. 8—9 by Oculina varicosa. 



F A M iLY— M ADREPORITID^. 



In this family the animals live crowded together in 

 societies, forming a branched coral; and the cellular 

 substance of the animal is in general not so perfectly 

 filled with calcareous matter as in the preceding family. 

 In consequence of this, we find the species to he of a 

 more spongy or a lighter texture. It is, however, of 

 these that the coral reefs are chiefly formed. Mr. Dar- 

 win found on the edges or outer margins of the reefs 

 of the Lagoon islands, that a species of the genus 

 Porites was the most abimdant, and that it formed 

 great irregularly rounded masses from four to eight 

 feet broad, and about the same in thickness. Tlie 

 fringing reefs, again, of the Mauritius had their outer 

 edges formed of great shapeless masses of branching 

 Madrepores, belonging chiefly to two species, the Mad- 

 repora cort/mbosa and M. pocilUfera, 



Family— MILL EPOPJD^E. 



According to Mr. Nelson, the animals forming the 

 corals belonging to this family are very difi'erent 

 from those of any other coral, being quadrangular, 

 expanded at intervals into four rays, and destitute 

 of any true tentacles. — [Gray.) The greater bulk of the 

 described species of millepores are fossil. A few, how- 

 ever, are recent ; and one of these {^MiUep)ora compla- 

 nata) is said by Mr. Darwin to be next to Porites in 

 abundance, as forming the reefs of the Lagoon islands. 

 This species appears to possess the stinging or nettling 

 power possessed by the Actinije. Lieut. Burnaby, 

 R.N., informed the writer of this, that at the Cocos or 

 Keeling islands he had often experienced this fact. 

 Mr. Darwin, at the same locality, says that he found 

 both this species and Mitlepora aleicornis to possess 

 this power. The family is represented in Plate 1, 

 fig. 10, by the species MUlcpora spotigHcs. 



Sub-class II.— POLYZOA or BRYOZOA (Molluscan Zoophytes). 



Bv many naturalists this sub-class of Zoophytes is 

 placed in the sub-kingdom Mollusca. In some respects, 

 as we have observed already at the conclusion of our 

 article MoLLUSCA, the species are closely allied to 

 those of the class Tunioata, more especially the family 

 Botryllidce or Compound Ascidians ; and in others they 

 approach the Terehratulce amongst the Brachiopoda. 

 The Polyzoa are compound animals, and the growth 

 formed by the associated animals or colony, is termed 

 " polyzoary," corresponding with the " poU'pidom" of 

 the Anthozoa. This is formed of a number of little 

 chambers, or cells, organically united, each of which 

 contains a polypide. They possess organs of digestion, 

 respiration, and circulation, have a muscular and 

 nervous system, and in an embryonic state are fur- 

 nished with organs of motion. Their reproduction is 

 elfocted in three dilferent modes ; b}' gemmation, by 

 ova, and by what have been called free locomotive 

 embryos. 



The numerous species of animals belonging to the 



class Polyzoa have been arranged in two orders, accord- 

 ing to the disposition of the tentacles which surround 

 the mouth — 1st, Those which have the tentacles dis- 

 posed in a ring surrounding the unarmed mouth — all 

 natives of the sea, the Infimdihulata ; and 2nd, Those 

 in which the tentacles are disposed in a crescent, or 

 horse-shoe shape — all natives of fresh water, the Hippo- 

 crepia. 



Order I.— INFUNDIBULATA. 



This order is by far the most extensive in number. 

 The animals have the tentacles disposed in an unin- 

 terrupted series round the annular disc surrounding the 

 unarmed mouth. This order is represented in Plate 1, 

 fig. 8, by Bicdlaria fastigiata ; in fig. 7, by Cellcpora 

 hyalina; and in fig 11, by Serialaria lendigera. 



Order IL— HIPPOCREPIA. 



This order contains only, comparatively speaking, 



