SECTION III 

 PHYLUM AND CLASS PORIFERA. 



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 unicellular animals, and the Metazoa or multicellular 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. 18), 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. 



1. EXAMPLE OF THE CLASS Syecn yckttitwstcm. 



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. 69), all con- 

 nected 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 8. ci/iafinn, a common European species 

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



