24 ON CLASSIFICATION. 



as the blood of the move highly organized animals. The gastric 

 chamber of the Actinozoa does not lie free in the interior of the 

 body, but is connected to the sides of it by means of mem- 

 branous partitions, the so-called "mesenteries" (/), which pass 

 radially from the stomach to the side walls of the body, and so 

 divide the " perivisceral cavity " into a number of chambers, 

 which communicate with the bases of the tentacles. In the 

 whole of the Hydrozoa the reproductive organs were attached to 

 the exterior of the body, and projected from it. In the whole 

 of the Actinozoa, on the other hand, the reproductive organs 

 (of which both sexes are frequently combined in the same indi- 

 vidual) are internal, inasmuch as they are situated in the 

 substance of the mesenteries (g). 



These are the universal and distinctive characters of the 

 Actinozoa. That some are simple and some are compound 

 organisms ; that some are fixed and some free swimmers ; that 

 many are soft, while a great number are provided with very 

 dense skeletons ; that some possess a rudimentary nervous 

 system, while the majority have as yet afforded no trace of any 

 such structure, are secondary circumstances in no way affecting 

 the problem before us, which is, to find a diagnostic definition of 

 the group. 



Notwithstanding the invariably minute size of the organisms 

 which constitute the next class on the list — the Polyzoa — they 

 exhibit a very great advance in complexity of structure. In 

 such a compound Polyzoon as the Sea-mat, or Fhistra, the entire 

 surface of the foliaceous expansion, on being examined by the 

 microscope, will be found to be beset with an infinitude of 

 minute apertures leading into little chambers, out of each of 

 which, when the animal was living and active, multitudes of 

 little creatures might be seen protruding the oral extremities 

 of their bodies. The ends of the branches of the freshwater 

 genus PlumateJIa, represented in Fig. 6, present a similar spec- 

 tacle. Each mouth is surrounded by a circlet of tentacles ; and, 

 as every tentacle is fringed with long and active vibratile cilia, 

 lashing the water towards the mouth, hundreds and thousands 

 of little Maelstroms are created, each tending to suck down 

 such nutritious bodies, living or dead, as come within its range. 



