PHOTO-MICROGRAPHY. tS 



remain spread out in the form of an expanded ring, their surface 

 is covered with ciHa in ever active movement. These ciHa create 

 a perfect whirlpool, having the mouth as its centre, and any object 

 small enough to be swallowed is brought in quite as effectually as 

 by the less refined, and apparently more energetic, seizure by the 

 tentacles of the Hydrozoa. As may be supposed, the tentacles 

 of these latter do not possess cilia. 



It remains to add that both classes, as well as being abun- 

 dantly represented by living forms, are also found fossilized. The 

 Hydrozoa have representatives as old as the lower Silurian. The 

 Polyzoa also certainly date as far back as that, and possibly (if 

 the Oldhamia belong to this class) as far as the Cambrian \ 

 but the exact zoological position of this interesting fossil is doubtful. 



Coming to comparatively recent geological periods, one genus 

 of Hydrozoa (viz., Hydraditiid) still found living, has been 

 observed in the chalk. Otherwise, Hydrozoa, common and 

 varied as are their living forms, have not been found fossil. 

 Perhaps this is because their skeletons are more perishable than 

 those of the Polyzoa. 



The skeletons of the other class, the Polyzoa, which are 

 generally calcareous (or formed of chalk), are very numerous in 

 the Devonian, Carboniferous, Permian, Triassic, Cretaceous (very 

 abundant), and in the Miocene and Pliocene (as the Suffolk crag) 

 very numerous. Some of the species found in the latter forma- 

 tion are identical with existing species; others of them have 

 disappeared. 



As regards recent and living forms of both classes, it is well to 

 state that while there are a few Hydrozoa occurring in fresh water 

 (as in the several species of Hydra), and also a few Polyzoa 

 (belonging to a special sub-division of the class in which the 

 tentacles are arranged in the form of a horse-shoe), yet the vastly 

 greater portion of both classes are marine. 



lpboto=^fHMcroorapb^» 



By Harry Barker. 



WITHOUT some knowledge of the ordinary wet and dry 

 photographic processes, it is utterly useless to attempt to 

 photograph with the microscope. For to a skilled 

 PHOTOGRAPHER the inherent difficulties can only be overcome by 

 employing the best materials, adjusting the apparatus with the 



