136 



ACTINOZOA. 



arrangement has been noticed 



. ,. , , , in the delicate 



muscles which surround the tips of the tenta- 

 cula. 



Though, histologically, the several structures of 

 Actinia admit of being resolved into two founda- 

 tion membranes, an ectoderm and an endoderm 

 yet each of these, more especially the former 5 

 manifests a tendency to differentiate into other 

 secondary layers, so that several apparently dis- 

 tinct tissues are recognizable in the body of the 

 adult animal. This is well seen in the column 

 wall, the principal thickness of which is composed 

 of the two sets of muscular fibres mentioned above. 

 That portion of the ectoderm which serves as an 

 external investment to this muscular wall appears 

 to consist, in some Actiniae at least, of two separ- 

 able, transparent membranes ; an outer, or epithe- 

 lial^ forming the general surface of the body, and 

 an inner or dermal layer in immediate contact 

 with the muscular substance. The dermal mem- 

 brane is almost wholly made up of a structureless 

 periplast containing very few endoplasts ; in the 

 epithelium, however, endoplasts are more abundant. 

 Between these two membranes thread-cells are 

 sometimes found embedded in such numbers as 

 almost to form a true layer, while close beneath 

 the epithelium occur masses of the pigment gran- 

 ules, to which the varied, and often gorgeous, 

 colours of these animals would seem to be due. 

 The endodermal lining of the muscular wall is, 

 in like manner, composed of two membranes, the 

 one superficial, the other in direct contact with 

 the deeper longitudinal fibres. Both ectoderm 

 and endoderm have their free surfaces more or 

 less abundantly ciliated. The structure of the 



