RADIATA. 



703 



Boft tissues of the animal, no longer participates in its vitai operations, and may be almost said to bo dead ; in 

 fact the gelatinous flesli is frequently withdrawn from the lower part, so that it remains as an inert stony mass, 

 whilst the upper portion is actively growing. In some of the arborescent corals ( MadreplujUidcc), the stony matter 

 appears to be entirely deposited in the substance of the polypes themselves; which are seated only at the ca;- 

 tremiti.es of the branches ; but in the more massive species ( Afadreporidcc ), it is deposited also in the connecting 

 gelatinous flesh, and the polype-cells are scattered over the entire surface. 



The Order may be divided into the following families :— 



Section A. Body coriaceous or fleshy. 



Family 1. Aotiniadce. Polypes separate and single. 



Family 2. Zoanthid-ce. Polypes gemmiparous, and associated by a common base. 



Section B. Body secreting a calcareous polypidom. 



Family 3. Madrcphi/Uidcc. Coral with terminal cells. 



Family 4. Madreporldee. Coral cellular throughout, the cells connected by a calcareous network, their 

 own walls also being porous. 



All those massive corals, to which the formation of coral reefs and islands is chiefly due, belong to the second 

 section of this order ; and most of them to the family MadreporidxE. Whilst the animals of the first section are 

 abundant in nearly all latitudes, those of the second are at present almost entirely restricted to tropical seas. 

 The large amount of coral limestone , however, found interposed amongst various other stratified rocks, from the 

 oldest even almost to the most recent, shows that they must have formerly had a much more extensive distribu- 

 tion. Only one small species (PocUlopora inter stiiicta) belonging to the third family, and Wiree ( Turhinolia bo- 

 realls, T. ■mlUctia.na, and Caryopliyllia 5mi(Aii_) belonging to the fourth, have been until recently knuwn in British 

 sens. At the last meeting of the British Association, howL'ver, Mr. Slac Andrew announced the very inter- 

 esting discovery of a living Fungla dredged up off the coast of Zetland. 



Obdeu III.— ASTEROIDA. 



— alcyoman Pot.tpf.. 



Thi'i division receives its designation from the star-like appearance of the short thick tentacula, six or eight in 



number, when expanded around the mouth. These tentacula are unpro- 

 vided with cilia ; but a number of little projections may he seen along their 

 margins, which probably increase their prehensile power. The mouth 

 leads to a stomach, which is suspended in the midst of the general cavity 

 of the body by partitions radiating from its walls ; the number of these 

 partitions, and consequently that of the chambers surrounding the 

 stomach, being the same with that of the tentacula. Instead of being 

 closed at its lower extremity, however, like that of the Actiniform polypes, 

 the stomach of the Alcyonians opens into the canals that ramify through 

 the fleshy mass in which they are imbedded ; the orifice being surrounded 

 by a circular mu?cle or sphincter, by the actions of which it may be ex- 

 panded or entirely closed. The chambers which surround the stomach 

 communicate above with the cavity of the tentacula, each of which has 

 a small orifice at its extremity ; whilst below they are continuous with 

 the ramifying canals just mentioned ; and the membranous septa which 

 support the stomach do not cease at its lower extremity, hut are prolonged downwards as plaits or folds of the 

 lining of these canals, until they gradually disappear. Here, too, the life of the individual polypes is subordinate 

 to that of the general mass ; and it is from the latter that all 

 the extensions of the fabric by gemmation take place. On 

 the other hand, the ova are developed in the substance of 

 the membranous folds, and make their way outwards 

 through the mouth. 



In this order we find a remarkable diversity in the charac- 

 ter of the polypidom. together with a great general similarity 

 in the structure of the polypes themselves. It contains no 

 solitary species ; and the essential character, by which it is 

 most distinguished from the Ilelianthoida, is the intimate 

 connection of the individuals of the same ma^s. In the com- 

 mon Alcyoiiium, the polypidom has something of a spongy 

 texture ; being composed of a gelatinous flesh burrowed by 

 a network of canals, and strengthened by a multitude of 

 spicules of mineral matter, which form a sort of loose .skeleton 

 that extends throughout the mass, especially strengthening 

 its surface. In the Tublpora musica, or organ-pipe coral, the 

 external integument of each polype is completely consoli- 

 dated into a calcareous tube. On the other hand, in the Hed Coral, it is the centre which is thus hardened, 

 forming a very dense stony axis, on the smooth surface of which not a vestige of polj-pe-cells can he detected. 

 This axis is clothed in the living state with a gelatinous flesh that is channelled out, like that of the Alcy<yniu7ii^ 



Fio. 17.- 



YoniUjM ; A, portion enhrt'd, ^ 

 the l'oly,.es. 



