1 8 THE ANTHOZOA 



solenia, containing spicules and spicule- forming cells, and into 

 this mass the proximal moieties of the zooid cavities extend. 

 This line of differentiation culminates in the Nephthyidae. 



Starting again from the Cornulariidae, we get another line of 

 differentiation, culminating in the Pseudaxonia. As in the first 

 case a fusion of cushion-like thickenings at the bases of the zooids 

 results in the formation of a stout, crustaceous coenenchyme. But 

 the vertical growth of the colony, instead of being arrived at by 

 elongation of the individual zooids and their aggregation into 

 bundles, is effected by the upgrowth of the creeping coenenchy- 

 matous expansion, which deserts the surface of attachment and 

 expands in the water. In this condition one surface of the colony 

 represents the attached surface of an encrusting form and is sterile, 

 the other face bears the exsert distal moieties of the zooids. For 

 mechanical reasons the colony does not retain its flattened form, 

 but becomes rolled up like a paper spill ; the sterile portion forms 

 the interior of a hollow cylinder, and the fertile portion is external. 

 By the excessive development of spicules on the internal (primi- 

 tively attached) surface, the colony becomes differentiated into a 

 softer cortical layer and a denser axial mass, both being penetrated 

 by numerous solenia. The axial mass, hollow at first, becomes 

 solidified in higher forms, and then it may either consist of closely 

 interlocked but distinct spicules, imbedded in a mesogloeal matrix 

 which is penetrated by solenia, as is the case in the Briareidae, or 

 the axis may consist of closely interlocking spicules, imbedded in 

 a mesogloeal matrix which is surrounded but not penetrated by 

 solenia, as in the Sclerogorgidae, or the spicules may be fused 

 together so as to form a dense calcareous axis which is not 

 penetrated by solenia, as in the Corallidae. 



A third line of differentiation gives rise to the division 

 Axifera. In this case the vertical extension of the colony is 

 effected by the formation of a horny secretion between the 

 primitively crustaceous colony and the surface of attachment. The 

 horny secretion, growing rapidly in thickness by the superimposi- 

 tion of new layers, raises the colony up in the water, and 

 presently, by continual growth at the summit, the horny matter, 

 which at first was basal, comes to form an axis, supporting the 

 colony by which it is encrusted like a tree by its bark. The axis 

 may branch in various ways, and may become partly calcified, and 

 thus we get the various forms of the Dasygorgidae, Isidae, 

 Primnoidae, and Gorgonidae. 



A fourth line of differentiation leads to the Pennatulidae. 

 The starting-point from the Cornularian ancestor is probably to be 

 found in the genera Telesto and Coelogorgia. In this case 

 vertical extension is attained by the extreme elongation of a 

 single zooid which, as it grows upwards, gives off solenia from 



