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^ LIBRARY, 

 Class II.— PORIFERA (Sponges). ^ '^^i: ' 



Characters. — (1) Porifera, or Sponges, are the lowest Metazoa; 

 (2) a sponge is a cup-like structure with a wall consisting of three layers 

 outer, middle, and inner — all in apposition to one another; (3) the inner 

 cells are ciliated; (4) the middle layer mostly develops a skeleton of lime, 

 horn, or flint; (5) the walls of the body are perforated by innumerable small 

 holes, through which water is taken in for nutritive purposes — the water 

 is washed along channels in the hody walls by the inner ciliated cells, 

 and flows out through at least one large aperture (osculum) ; (6) a sponge 

 "physiologically is a mere colony of Protozoa in which individual members 

 are subordinated to the good of the whole." 



Class III.— CCELENTERATA (Hollow-bodied 



Animals). 



Characters. — Coelenterates are Metazoa — (1) In which there is neither 

 a distinct alimentary canal, nor a distinct body cavity, but a single cavity 

 which serves for both ( hence coelenterates are practically permanent gastndce) ; 

 (2) they exhibit radial symmetry ; (3) \h.ey YisMe stinging cells ; (4) there are 

 organs of prehension (tentacles) around the mouth. 



SXJB-DIVISIONS. 



I. HYDROZOA (Hydra-like). — ^ Coelenterates, of which a transverse 

 section just behind the mouth shows a simple tube with a double layered 

 wall (ectoderm and endoderm). Hydrozoa include Coelenterates, which 

 may be roughly said to assume four different forms. — (1) Hydriform through- 

 out life (e. g. hydra) ; or (2) medusif orm throughout life ; or (3) colonial 

 animals, partly hydriform and partly medusiform (e. g. seafirs) ; or (4) hydri- 

 form in young stage, and medusiform in the adult {e.g. aurelia, jelly-fish). 

 Types (3) and (4) exhibit alternation of generations, i. e. there is a successive 

 alternation of sexual and asexual animals related as parent and offspring. 



[A hydrozoon is said to be hydriform, when, like hydra, it is a simple 

 sac, fixed at one end and open at the other (mouth end), around which is the 

 circle of tentacles. It is said to be medusiform, when (like aurelia) it is a 

 free swimming sac in the form of a bell with a clapper, or of an umbrella 

 with a much thickened handle, the mouth being placed at the end of the 

 clapper of the bell, or of the handle of the umbrella.] 



II. ACTINOZA (Actinia-like). — Coelenterates, in which a transverse 

 section behind the mouth usually shows a double tube — one within the other, 

 and separated from one another by six or more longitudinal partitions 

 (mesenteries). The inner tube extends only a short way down from the 

 mouth, and forms a rudimentary sort of oesophagus. Never alternations of 

 generations. Usually colonial forms, and with a skeleton of lime (corals). 

 e.g. Actinia (sea-anemone), which however is not colonial. 



III. CTENOPHORA (comb-bearers). — Coslenterates, with 8 branches 

 from the "alimentary canal." of comb-like appearance. A small group 

 forming a link between Coelenterates and Worms. 



