^12 



OUR COMMON- BRITISir 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 common 

 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 



Fig. 168. — Cells o{ Membranipora 

 (magnified, 60). 



shadow, is quite sufficient 

 to cause these zoophytes 

 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 Botrylhis), 

 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 



