212 



OUR COMMON BRITISH FOSSILS. 



These are the ciliated tentacles, and a higher power 

 (say a half-inch objective) will show the cilia actively 

 at work on the lobe-like tentacles, so that the sea- 

 water is thrown into a state of microscopical commo- 

 tion thereby, and particles of floating matter are seen 



whirled about, and finally 

 gathered by the vortex into 

 the mouth of one of the 

 little creatures, whose dia- 

 phanous body enables us 

 to trace its passage to the 

 stomach. The slightest jerk, 

 or even the falling of a 

 shadow, is quite sufficient 

 to cause these zoophytes 



Fig. i68.-ceiisof^*m/>r to withdraw within the pro- 

 tection of their cells to 

 reappear immediately afterwards. 



All of the Polyzoa are really highly organized 

 creatures, possessed of a mouth, stomach, intestinal 

 canal, anus, and nervous and muscular systems. They 

 are very nearly related to the Ascidia or " sea-squirts " 

 (especially to those social groups of the latter family 

 we see clustered on seaweeds, such as Botryllus). 

 The absence of any solid parts in the " sea-squirt " 

 family has of course prevented our finding any traces 

 of them in the fossil state. This is to be regretted, 

 especially as the tadpole-shaped young of these 

 Ascidians so much resemble the internal organization 



