RADIATA. 



703 



soft tissues of the animal, no longer participates In its vitai operations, and may be almost said to be dead ; ia 

 fact the gelatinous flesh is frequently withdrawn from the lower part, so that it remains as an inert stony mass, 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 (ifadrephyllidce), the stony matter 

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

 tremitiet of the branches ; but in the more massive species (Madreporidw), 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, Actiniadm. Polypes separate and single. 



Family 2. Zoanthidce. Polypes gemmiparous, and associated by a common base. 



Section B. Body secreting a calcareous polypidom. 



Family 3. MadrephyUidce. Coral with terminal cells. 



Family 4. MadreporidciE. 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 Madreporidce, 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 (Podllopora interstincta) belonging to the third family, and three ( Turbiiwlla bo- 

 realis, T. miUetiana, and Car ijophyUki Smithii) belonging to the fourth, have been until recently known in British 

 seas. At the last meeting of the British Association, however, Mr. Mao Andrew announced the very inter- 

 esting discovery of a living Fungia dredged ui> off the coast of Zetland. 



Obdeb III.— ASTEROIDA. 



16.— Alcyunu 



Thii 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 be 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 muscle or sphincter, by the actions of which it may be ex. 

 pauded or entirely closed. The chambers which surround the stomach 

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

 a small orifice at its extremity ; whUst 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, but 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 

 tho membranous folds, and make their way outwards 

 through the mouth. 



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

 ter of tho 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 tho Helianthoida, is tho intimate 

 connection of the individuals of the same mass. In the com- 

 mon Alcyonium, the polypidom has something of a !:pongy 

 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 tho Tuhipora 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 Red Coral, it is the centre which is thus h.irdcncd, 

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

 This axis is clothed in the living state with a gelatinous flesh that is channelled out, like that of the Aleyonium, 



-ALcyONIUMi A, portion enlarged, 

 the Polypc.-t. 



