ACTINOZOA. 83 



CHAPTEE IX. 



ACTINOZOA. 



The second great class of the Codenterata is that of the Aetinozoa, 

 comprising the. Sea-anemones and their allies, the Corals, the Sea- 

 pens, the Sea-shrubs, and various other organisms. They are all 

 defined as Coelenterate animals in which there is a distinct digestive 

 sac which opens helmo into the general cavity of the body, but is 

 nevertheless separated from the body-walls by an intervening space 

 {the "perivisceral space "), which is divided into a number of vertical 

 compartinents by a series of partitions or ''^mesenteries," to the faces 

 of which the reproductive organs are attached. The Aetinozoa (fig. 47), 

 therefore, differ fundamentally 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 perivisceral space. The result of this is, that 

 whilst the body of a HydrozoSn exhibits on transverse section a 

 single tube only, formed by the walls of the combined digestive and 

 somatic cavity, the body of an Actiiiozoon exhibits two concentric 

 tubes, one formed by the digestive sac and the other by the general 

 walls of the body. Further, in the Aetinozoa 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 Aetinozoa differ little 

 from those of the Hydrozoa. The body is essentially composed of 

 two fundamental layers — an ectoderm and endoderm ; but there are 

 often well-developed layers of muscular fibres, somewhat obscuring 

 this simplicity of structure. Thread-cells are most commonly pres- 

 ent in abundance. Cilia are very generally developed, especially in 

 the endoderm lining the body-cavity, where they serve to maintain 

 a, circulation of the contained fluids. The only digestive apparatus 

 consists of a tubular or sac-like stomach, which opens inferiorly 

 directly into the body-cavity (fig. 47), and communicates with the 

 outer world through the mouth. Definite nerve-centres do not 



