CHAPTER IX. 



ACTINOZOA. 



THE second great class of the Coelenterata is that of the 

 Actinozoa, comprising the sea-anemones and their allies, the 

 corals, the sea-pens, the sea-shrubs, and various other organ- 

 isms. They are all defined as Ccelenterate animals in which 

 there is a distinct digestive sac which opens below into the 

 general cavity of the body, but is nevertheless separated from 

 the body-walls by an intervening space, which is divided into 

 a number of vertical compartments by a series of partitions 

 or " mesenteries" to the faces of which the reproductive organs 

 are attached. The Actinozoa (Fig. 12), therefore, differ fun- 

 damentally from the Hydrozoa in this, that whereas in the 

 latter the digestive cavity is identical with the body-cavity, 

 in the former there is a distinct digestive sac, which opens 

 truly into the body-cavity, but is nevertheless separated from 

 it by an intervening space. The result of this is, that while 

 the body of a Hydrozodn exhibits on transverse section a 

 single tube only, formed by the walls of the combined diges- 

 tive and somatic cavity, the body of an Actinozodn exhibits 

 two concentric tubes, one formed by the digestive sac and the 

 other by the general walls of the body (Fig. 25, A). Further, 

 in the Actinozoa the reproductive organs are always internal, 

 and are never in the form of external processes of the body- 

 wall as in the Hydrozoa. 



In their minute structure the tissues in the Actinozoa dif- 

 fer little from those of the Hydrozoa. The body is essen- 

 tially composed of two fundamental layers an ectoderm and 

 endoderm ; but there are often well-developed layers of mus- 

 cular fibres, somewhat obscuring this simplicity of structure. 

 Thread-cells are most commonly present in abundance. Cilia 

 are very generally developed, especially in the endoderm lining 



