II COELENTERA TA 31 



to a remarkable degree the power of regeneration of lost 

 parts. If it is cut in two transversely, the bottom half will 

 form a new mouth and tentacles, whilst the top half will form 

 a new base ; in fact, even quite small pieces of the body are 

 said to be capable of reproducing the whole. 



In such a form as Hydra, the specialisation 



^^of^CeUs''*'^ of cells into separate tissues is most striking. 



The differentiation of skin cells, digestive cells, 



dart cells, muscle processes, nerve cells, eggs and sperms, all 



mark the much higher stage of evolution of this form, as 



compared with even the most complex of the Protozoa. On 



-the other hand, the form and behaviour of the egg cell as it 



matures, strikingly recall the Amoeba. The digestive cells 



also, with their power of throwing out irregular processes to 



engulf food, or whip -like processes for lashing the water, 



resemble many Protozoa, and indicate the primitive nature of 



these simple multicellular forms, and their close affinity with 



the Protozoan organisms from amongst which they must 



have taken their origin. 



Colonial Hydra-like forms. 



There are many genera which — though they begin life as 

 a, single individual or polyp very similar to Hydra — produce, 

 by lateral budding, a large colony of polyps, all united by a 

 common branched stem which is permanently attached to 

 one spot. 



Many of these colonial forms also secrete round their 

 common stem, and round each branch, a horny, tubular, 

 protective covering, which may stop at the bases of the 

 separate polyps, or may form cups extending right to the 

 bases of the tentacles, so that each polyp can be completely 

 withdrawn into its cup for protection. 



Colonial The yellow, sandy-looking Sea Firs, which are 



Forms with frequently picked up on the shores of our rocky 

 horny Cups, coasts after stormy weather, and are commonly 

 mistaken for seaweeds, are examples of such colonial hydroids 

 in which horny protective cups are present. In one of these, 

 Sertularia, the polyps are set close on each side of the axis 

 (Fig. 13), which may be much branched and arises from a 

 creeping stolon attached to a roek or shell. In Antennularia 



