118 THEIR DIFFERENT CLASSES. 



classes, the Anthozoa, which have a body capable of 

 contraction in every part, and perfectly symmetrical, 

 with but a single aperture for the entrance of food ; and 

 the Polyzoa (or Bryozoa), whose bodies are unsymmetri- 

 cal, and incapable of contraction, while they are fur- 

 nished with a separate mouth and vent. The first are 

 obviously akin to radiate animals, while the latter show 

 a close resemblance in structure to the simpler members 

 of the Mollusca. The Polyzoa, though of much smaller 

 size than many of the Anthozoa, are much more perfectly 

 organized, and of a higher type in animal existence. In 

 the compound Anthozoa the individuality of the Polypes 

 is not clearly maintained, but each is, as it were, a bud 

 issuing from a common fleshy trunk, of similar sub- 

 stance ; while in the Polyzoa every individual is distinct 

 within its own precincts, though connected, like the 

 Siamese twins, by a common band. Dr. Johnston aptly 

 compares the former to " a chain of which all the links 

 are welded ;" the latter, " to a necklace, where the beads 

 are strung together by a common thread." The Antho- 

 zoa are divided by Dr. Johnston into three orders, easily 

 recognisable by the nature of the skeleton ; the 1st, 

 Hydroida, having Polypes enclosed in horny, tubular, 

 plant-like sheaths, forming an external covering to their 

 trunk ; the 2nd, Asteroida, a calcareous or horny axis, 

 or internal skeleton, surrounded by the fleshy parts of 

 the compound body; and the 3rd, Helianthoida, having 

 a calcareous or coriaceous skeleton composed of plates, 

 radiating, like the gills of a mushroom, towards a com- 

 mon centre. The British Asteroida being all natives 

 of the deeper parts of the sea, will more properly be 



