ACTINOZOA 



"5 



lying inside the theca and its flap-like extension which overhangs the 

 lip and reaches for some distance down its outer surface. It is the 

 interruptions in which lie these tubular connexions that show as 

 foramina in the dry skeleton. 



The AcTiNOZOA, exemplified by the Alcyonaria and the Zoantharia, 

 are clearly marked off from the Hydrozoa by a number of features 

 which they possess in common. Their general form of body is of the 

 polyp type : there is no medusoid phase in their life-history. The 

 individual polyp is larger in size and more complex in structure than 

 is that of the Hydrozoon. The part of the body corresponding to the 

 oral cone of the Hydrozoon is turned inwards so as to hang down in the 

 coelenteric cavity as the stomodaeum. The stomodaeal wall is sus- 



FiG. 53- 



Mushroom Coral — Fungia. A and B, young fixed stages ; C, adult condition, 

 after Bourne.) 



(A and B 



pended from the body-wall by the mesenteries — thin vertical partitions 

 of mesogloea, the deep recesses between which are lined with endoderm. 

 The endoderm on the face of the mesentery develops prominent longi- 

 tudinal bands of muscle : it also develops the gonad. Finally, when a 

 skeleton is present this is normally not in the form of thickened cuticle 

 (perisarc) but in the form either of isolated spicules secreted in the meso- 

 gloea by immigrant cells from the ectoderm or of a mass of secretion 

 formed underneath the ectoderm of the base. 



The phylum Coelenterata, the more important subdivisions of 

 which have been dealt with in this chapter, includes what are on the 

 whole the most nearly primitive members of the Metazoa. They are 

 above all characterized (i) by the possession of a general internal cavity, 

 the coelenteron, which has not yet become subdivided into a separate 



I I 



