[73] 



1b?bro3oa anb pol?3oa* 



By Dr. G. D. Brown, President. 



IN giving a short account of these, it is necessary to state that 

 while specimens of each of the above classes have certain 

 points of agreement, which will be spoken of presently, the 

 two classes are so different in their anatomical structure that 

 there is really a nearer relation between a dog and a fish than 

 between one of the Hydrozoa and one of the Polyzoa. Thus, 

 while the Polyzoa are comparatively highly organized, and some- 

 what complicated in structure, forming a division of the sub- 

 kingdom MoUusca, which comes next below the sub-kingdom 

 Vertebrata, — having a mouth to take in food, with oesophagus, 

 stomach, and intestine, a separate aperture for the getting rid of 

 undigested matter, and nervous and reproductive systems ; the 

 other class with which we propose to contrast them^ namely, the 

 Hydrozoa or Hydroida, has a much simpler structure, and occu- 

 pies a far lower position in the animal kingdom. It belongs to 

 the sub-kingdom Ccelcnterata, of which it is the first class ; the 

 second class being the Actinozoa, or corals, sea-anemones, etc. 

 There are two other sub-kingdoms, namely, that which includes 

 the Radiata, consisting principally of the sea-urchins, star-fishes, 

 etc., and one next higher than this, which contains the worms, 

 the Crustacea, and the insects ; all these are placed below the 

 Polyzoa, or Bryozoa, as the class is generally named by most 

 continental authorities. 



The Hydrozoa, then, or Hydroida, like the Actinozoa, which 

 include the corals, sea-anemones, and others, have a most simple 

 structure, such as many of us have seen in the sea-anemone, or 

 the common Hydra of our ponds and ditches. In these cases, 

 each is a distinct animal, and is a bag with only one orifice to 

 receive the food, — this orifice being surrounded by a ring of 

 tentacles which have the power of grasping, and often of para- 

 lyzing, the objects constituting its food, and bringing them into 

 the mouth. After being swallowed, the undigested portions are 

 returned by the way they entered. It appears to be a matter of 

 indifference how these simple forms of Hydrozoa are treated. 

 Turned inside out, what was before the outer skin acts very well 

 as a digestive stomach, and if an individual be cut into pieces, 

 each piece starts on its own account, and becomes an individual, 

 perfect in all its parts. 



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