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Show 24 ON CLASSIFICATION. as the blood of tho 1noro highly organized animal A. The gastric chamber of tho Aotinozoa rloes not lie free in the interior of tho body, bnt is connected to tho sides of it by moans of membranous partitions, the so-called "mesenteries" (f), which paRR radially fron1 the stomach to the side walls of tho body, and so divide the "perivisceral cavity " into a number of ohan1bon:;, which com1nunicate with the bases of the tentacles. In tho whole of the Hydrozoa the reproductive organs wore attachocl to the exterior of the body, and pr~jected fro1n it. In the whole of the Aotinozoa, on the other hand, the reproductive organR (of which both sexes are frequently combh1ed in tho Aamo individual) are internal, inasmuch as they are situated in tho Rubstance of the mesenteries (g). These are the uni versa I and distinctive characters of tho Aotinozoa. That some are simple and some are con1ponnd 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 rudi1nentary 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 organis1ns 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 Flustra, the entiro surface of the foliaceous expansion, on being examined by tho microscope, will be found to be Leset with an infinitude of minute apertures leading into little cha1nbers, out of each of which, when the animal was living and activo, 1uultitudes of little creatures might be seen protruding the oral extremities of their bodies. The ends of the branches of the freshwater genus Plumatella, represented i:n Fig. 6, present a sin1ilar spectacle. 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. THE POLYZO.\. 25 Tho mouth (Fig. 8) leads into a lono· and wide pl1aryng nl and oosopbagoal tube, which opens, below, into a <1 finit tomach. From this is continued a eli tinct intestin0, which bonds npon Fig. 7. Fig. 7. - Plmnatclla 1'(')JCns, n freshwater Polyzoon, magnifi Pcl (aftrr Allmnn ). itRelf towards the oral end of tho body, so as to form a sharp angle, and then terminates upon the outer surface ncar tho mouth; so that we have hero, for the first ti1ue in our ascending survey of the .A.nimal Kingdom, an ani1nal possessing a complete intestine, not only structuraJly separated from tho general substance of the body, and provided with perman nt apertures, as in the Hydrozoa and Aotinozoa, but. completely shut off from the perivisceral cavity, and in direct communication only with the external medium. All the Polyzoa posses. a nervous system, the characters and position of which arc very well defined. It consists of a single ganglion (Fig. 8, w), placed between the oral and the anal apertures, and sending off nerves in various directions. It has been affirmed that, in some Polyzoa, there is a more extended system of nerves by which the various zooids of the compound orgauism aro placed in cmnmunication; but of that we want further evidence. In these animals no heart has been discovered as y t, tho matter which result from digestion percolating through tho walls of th . intestine, and becoming mixed with tho periviscOI'allluid. One of the structural characters which I have mentioned is exceedingly important. As I havo said, tho intesti11o is not straight, |