SECTION III 



PHYLUM AND CLASS PORIFERA [PARAZOA] 



THE microscopic animals described in the preceding section 

 are, as already repeatedly pointed out, characterised by their 

 unicellular character, and in this respect stand in contrast to the 

 remainder of the animal kingdom. The animal kingdom is thus 

 capable of division into two great subdivisions, the Protozoa or uni- 

 cellular animals, and the Metazoa or multicellular forms the latter 

 comprising all the groups that remain to be dealt with. In the 

 earliest stage of their existence all the multicellular animals or 

 Metazoa are, as already pointed out (p. 20), in a unicellular 

 condition, originating in a single cell, the fertilised ovum or 

 oosperm. By the process of segmentation or yolk-division the 

 unicellular oosperm becomes converted in all the Metazoa 

 into a mass of cells from which the body of the adult animal is 

 eventually built up. Of the Metazoa, the group which approxi- 

 mates most closely to the Protozoa is that now to be dealt with 

 the Porifera or Sponges. With all the other multicellular groups 

 the Sponges are so strongly in contrast that the Metazoa may 

 be regarded as falling into two main divisions the Porifera or 

 Parazoa, on the one hand, and all the rest of the Metazoa, grouped 

 together as Enterozoa, on the other. 



1. EXAMPLE OF THE CLASS Sycon gelatinosum. 



General External Appearance and Gross Structure. 



Sycon gelatinosum,' 1 one of the Calcareous Sponges, has the form of a 

 tuft, one to three inches long, of branching cylinders (Fig. 79), all 

 connected together at the base, where it is attached to the surface of 

 a rock or other solid body submerged in the sea. It is flexible, though 

 of tolerably firm consistency ; in colour it presents various shades 

 of gray or light brown. To the naked eye the surface appears 

 smooth, but when examined under the lens it is found to exhibit 

 a pattern of considerable regularity, formed by the presence of 



1 This species is an inhabitant of southern seas. In all essential respects 

 the account of it given above will apply to S. ciliatum, a common European 

 species which differs chiefly in the absence of the pore -membranes.. 



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