CNIDARIA 



157 



it is entirely closed. Cassiopeia is a semisedentary form, which lies 

 with its exumbrellar surface upwards on the mud of mangrove swamps. 

 The bell pulsates gently and brings in a constant stream of plankton 

 organisms which are seized by the lips. 



The mode of development described above is typical in the 

 Scyphomedusae. There are, however, certain exceptions. In the 

 genus Pelagia the medusa develops directly from the egg into an 

 ephyra larva, and in Cassiopeia the hydratuba only produces a single 

 ephyra at a time, a condition which is obviously primitive compared 

 with Amelia, *'polydisc" strobilization being a secondary adaptation 

 for the more effective spread of the species. 



Fig. 131. Diagrammatic longitudinal section through Pilema. Enteron and 

 its branches shown in black, many "sucking mouths" along the lips, can.r. 

 radial canal; sg.p. subgenital pit. 



Class ACTINOZOA (ANTHOZOA) 



Solitary or colonial coelenterates with polyp individuals only: 

 coelenteron divided by mesenteries: stomodaeum present: genital 

 cells derived from endoderm. 



They are divided into the two orders Alcyonaria and Zoantharia. 



Order ALCYONARIA 

 Actinozoa with eight mesenteries and eight pinnate tentacles; 

 stomodaeum with a single siphonoglyph (ciliated groove) ; skeleton 

 internal, consisting of spicules in the mesogloea, occasionally 

 supplemented by an external skeleton ; longitudinal muscles on the 

 ventral faces of the mesenteries. 



As a type of the order we will describe Alcyonium digitatum^ "Dead 

 men's fingers", a colonial form which occurs below low-tide mark, 

 attached to stones, in various sizes and shapes, but usually in broad- 

 lobed masses. A small portion or lobe of a colony is shown in Fig. 132, 

 and it is seen that the polyps project in life from the general surface 



