ACTINOZOA. 



247 



appear to consist principally, or only, of the thin primitive central lamella, 

 no secondary layer of stereoplasma, or but a very imperfect one, being- 

 developed. In such cases, the outer wall of the coral is also very thin 

 (fig. 128, b). Where secondary stereoplasma is well developed, the 

 thickened outer ends of the septa usually become fused with one another, 

 so as to form a more or less dense outer investment to the visceral 

 chamber. Sometimes (as in St?'eptelasma, fig. 127, b), the external wall 

 of the corallum seems to be formed wholly by the thickened outer ends 

 of the septa ; but in many forms the primitive theca can be recognised 

 in the substance of the outer investment as a thin dark plate (fig. 127, t) 

 formed by lateral outgrowths from the primordial septa. Hence in such 

 cases the peripheral ends of the septa continue to grow outwards subse- 



s' t s' s st s s' 



Fig. 127. — A, Portion of a cross-section of the recent Caryophyllia borealis, enlarged to show 

 the structure of the septa, b, A similar preparation of Streptelasma cornicidum, Ordovician, 

 N. America, c, A similar section of Zaphrentis Enniskilleni, Carboniferous, p, Dark line oc- 

 cupying the middle of the septum ; st, Layer of stereoplasma ; t, Theca, formed by out-growths 

 from the outer ends of the septa ; s, Septum of the first order ; s', Septum of the second order ; d, 

 Dissepiments. (Original.) 



quently to the formation of the original theca ; each septum thus coming 

 to consist of an intrathecal and an extrathecal portion. 



In some corals (as, for example, in Pholidophyllum), it can be clearly 

 shown that the septa are made up of calcareous trabecular or spines 

 (the " Vertical-leisten " of the Germans), which are directed upwards 

 and inwards towards the axis of the corallum, and which become united 

 with one another directly or by means of secondary sclerenchyma. In 

 Heliophyllum, Crepidophyllum, Phillipsastrcea, and other allied genera 

 of Palaeozoic corals, these oblique septal trabecular are greatly developed, 

 and are not obscured by the development of lateral layers of stereoplasma. 

 Hence in these genera cross-sections of the septa (fig. 128, b) exhibit 

 characteristic thickenings or cross-bars, which have been termed " car- 

 inas," and which are the result of the intersection of the septal spines 

 just spoken of, while the free edges of the septa are furnished with 

 pointed projections or teeth. 



