COMPARISON WITH CORAL POLYPS. 29 



probably food assimilation is localized, thus 

 suggesting a comparison with the stomach of 

 the polypite. 



To complete the sponge simile. Instead 

 of the horny tubing suited to the hydroid 

 form, there is an intricate network of horny 

 fibres, serving as a skeleton to support the 

 otherwise somewhat flaccid body-substance, 

 and in addition, the structure is rendered more 

 solid by the secretion of minute needles of 

 carbonate of lime or flint, in the flesh and fil)re. 

 These needles may be the homologue of the 

 calcareous skeleton of some hydroids, e.rj., a 

 foreign species of Hydractinia and the fossil 

 l^arkeria and others. 



Hydroids compared with the Sea- 

 anemones and the coral polyps. Higher 

 in the scale of development, above the 

 Hydrozoa, are placed the Sea-anemones and 

 the Coral polyps. In the hydroid animals the 

 stomach is a simple sack, but in the anemones 

 and coral polyps a slight though important 

 transformation has taken place. It is first 

 indicated in the higher medusae. Instead of 

 the simple sack-like stomach, the month 

 portion is turned inwards, {e.g. A certain kind 



