in. ANTHOZOA. 



251 



remain through which nourishment is talcen. fitomohphns* and Pohjdo- 

 nia frondom* (fig. 194) on coral banks in our warmer seas. 



Class III. Anthozoa (Actinozoa). 



Tlie Actinozoa, including the sea anemones, sea pens, and corals, 

 are exclusively marine. "With few exceptions they are sessile and 

 form colonies, often of enormous size. In this as in ajipearance 

 (fig. 109) they resemble the hydroid polyps. They have a pedal 



Fig. 199. — Antheomorpha elegaiis. s, .s, sagittal plane. 



disc, column, tentacles, and peristome with central mouth. They 

 are distinguished by their greater completeness in histological and 

 organological differentiation. The Anthozoan polyp has a well- 

 developed mesoglrea, the supporting layer of the hydroid being 

 here a layer of connective tissue with numerous cells, giving the 

 animals a tough fleshy consistency. Still more important as points 

 of distinction are the presence of an oesophagus and septaj bearing 

 mesenterial filaments and gonads. 



The mouth lies in the centre of the 2ieristome, and in sliajie is 

 usually oval or slit-lil^e. Hence there is a biradial symmetry — of 

 importance in the architectonic of the polyp — for there is a sagittal 

 axis (fig. 109, ,S', s) passing in the long axis of the mouth and a 

 transverse axis at right angles to it. From the mouth the ossopha- 

 gus hangs down into the body as a flattened tnl)e and ojiens at its 

 lower end into the wide gastrovascular cavity. In its development 

 this oesophagus is an inflected part of the peristome and hence lined 

 with ectoderm, and its lower end alone can be compared with the 

 mouth of the hydrozoan (fig. 200). 



The oesophagus is held in position liy radial partitions, the 

 septa, which stretch from base, column, and peristome to the 



