in. ANTHOZOA 



225 



they resemble the hydroid polyps. They have a pedal disc, column, 

 tentacles, and peristome with central mouth. They are distinguished 

 by their greater structural differentiation. The Anthozoan polyp has a 

 well-developed mesoglcea, tliis Ijeing a layer of connecti\'e tissue with 

 numerous cells, giving the animals a tough fleshy consistency. Still 

 more important are the oesophagus and septa; bearing mesenterial 

 filaments and gonads. 



The mouth, in the centre of the peristome, is usually oval or slit-like. 

 Hence there is a biradial symmetry for there is a sagittal axis (fig. 191, s, s) 



Fig. 



192 



Fig, 191. 



Fig. 191. — Antheomorpha ehgans. s, s, sagittal plane. 



Fig. 192. — Sagartia parasitica split lengthwise, a, acontia; c, septal canal; /, 

 mesenterial filaments; g, gonads; m, sphincter muscle; 0, oesophagus; p, peristome; r, 

 septa of different orders; j, siphonoglyphe; u<, cut wall of column. 



passing in the long axis of the mouth, and a transverse axis at right angles 

 to it. From the mouth the cesophagus hangs down into the body as a 

 flattened tube and opens at its lower end into the wide gastrovascular cavity. 

 This cesophagus 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. 192). It usually bears at either end a specialized 

 groove, the siphonoglyphe {s). 



The oesophagus is held in position by radial partitions, the septa (r), 

 which stretch from base, column, and peristome to the cesophagus, dividing 

 the peripheral part of the gastral space into small pockets, the radial 

 chambers, connected below the end of the oesophagus with the central 

 part. Above, these chambers continue into the tentacles. The tentacles 

 therefore are outgrowths from the radial chambers and usually equal 

 them in number. Besides the complete or primary septa which reach 

 15 



