18 COELENTERATA— CNIDARIA phylum ii 



may be composed of a bündle of styliform or twisted rods (Fig. 99), or of thin 

 lamellae. It extends from the floor of the visceral Chamber to the bottoni of 

 the calice, into which it projects for a greater or less distance. The structures 

 known as pali are narrow vertical plates which are inserted between the colu- 

 mella and the inner ends of the septa in one or more cycles (Fig. 99). 



The outer wall or theca is often formed by the secretion of a particular 

 ring-like fold of the ectoderm, and is constituted of distinct sclerites, having 

 separate calcification-centres, and connecting the outer borders of the septa 

 (euthecalia). In many cases the peripheral edges of the septa become thickened 

 and laterally fused to form a spurious theca (pseudothecalia) ; and occasionally 

 the dissepiments lying in a certain zone become united so as to form an inner 

 wall within the true theca. The epitheca is a usually smooth, sometimes corru- 

 gated, superficial calcareous investment, which, according to Koch, is merely a 

 Prolongation of the basal plate, and is secreted by the outer surface of the 

 ectoderm, which is reflected over the top of the corallum. The epitheca is 

 deposited either directly upon the septa, or upon the theca, or, when the septa 

 are produced outwards so as to form exothecal lamellae or ribs (costae), the 

 theca and epitheca are separated. Exothecal lamellae, not corresponding in 

 Position to the septa, are called pseudocostae or rugae. 



New individuals or colonies commonly originate by sexual reproduction. 

 Following fertilisation and segmentation of the ova, ciliated larvae are born, 

 which swim about for a time, become fixed, and develop into simple polyp 

 individuals. Vegetative or asexual increase by two sharply defined processes, 

 namely, hudding (or gemmation) and ^jssiow, assumes a great importance among 

 Anthozoans, resulting in the production of colonies or Stocks, often of large 

 size and exceeding complexity of form. 



New corallites are produced either within or without the calice of the 

 parent polyp. In extra-calicinal gemmation the buds are thrown out either 

 from the sides of the polyp (lateral gemmation), or are formed in the common 

 calcareous matrix which unites the various corallites of a colony {coenenchymal 

 and costal gemmation). In both cases the new corallites may diverge from one 

 another, being attached to the parent corallum only at the base, or they may 

 grow up closely opposed to the latter and to one another, so that the thecae 

 are in contact on all sides. In this way branched, dendroid or massive and 

 knob-like '("astraeiform") Compound coralla are formed. A less common 

 mode of increase is by hasal or stolonal gemmation. In this process the wall of 

 the original polyp sends out creeping prolongations (stolons) or basal expansions, 

 from which new corallites arise. In calicinal gemmation buds are produced 

 within the calice of the parent corallite, according to one or the other of the 

 following methods : either certain particular septa become enlarged and pro- 

 duced so as finally to enclose a new calicinal disk {septal gemmation) ; or tabulae 

 are produced upwards in the form of pockets, from which new corallites are 

 developed (tahular gemmation). In both septal and tabular gemmation, a portion 

 of the parent corallite including a part of the original wall is concerned in the 

 formation of buds ; while the septa or modified tabulae are converted into 

 portions of the new thecae, from which new septa then begin to grow inwards 

 towards the centre. 



A peculiar kind of calicinal gemmation is that known as rejuvenescence. In 

 this method only one bud is formed within the parent calice, but it enlarges 

 aintil it completelj ßlls the latter. By the indefinite repetition of this process, a 



