210 



CCELETTTERATA. 



and Medusce, and have included the former in the group of the 

 Ccdenterata. The Poi'ifera were for a long time taken for plants, 

 and more recently for Protozoon-stocks. While, however, the Polyps 

 and Medusae are distinguished as Cnidaria and are characterised by 

 the possession of nernatocysts and by the higher differentiation of their 

 tissues, the Porifera or tSpongiaria present more simple forms of 

 tissue in the spongy structure of their body, and are without nemato- 

 cysts. The entire structure of the body may, generally speaking, be 

 described as radial, although the radial symmetry does not appear in 

 most sDonges, and among the Cnidaria transitions towards lateral 



symmetry are ap- 

 parent. Similar 

 organs are usually 

 repeated round 

 the body axis four 

 or six times or in 

 multiples of these 

 numbers. 



Four distinct 

 types of body form 

 are met with in 

 the group Cmlente- 

 ,afa, viz., that of 



the Sponge; of the 

 Polyp ; of the Me- 

 dusa ; and of the 

 Ctenophorat 



The Sponge 

 type. The sim- 

 plest form of 

 Sponge is represented by a fixed cylindrical tube, with an exhalent 

 opening, the Osculum, at the free en4 (fig. 152). The contractile 

 wall is supported by skeletal spicules, and is pierced by numerous 

 inhalent pores, through which water and small food particles pass 

 into the ciliated internal space. By the fusion of separate indi- 

 viduals, and by reproduction by gemmation, the latter being the 

 more frequent mode, widely different Sponge stocks with compli- 

 cated canal systems are formed. The polyzooid nature of these is 

 made apparent by the presence of many oscula. 



The Polyp type. The Polyp has the form of a cylindrical or 

 piub-shaped tube, of which the posterior end is fixed and the opposed 



FIG. 152. -Young Si/ con (after Fr. E. Schulze). O, Osculum or 

 exhalent pore ; P, pore in the wall. 



