SECTION III. PHYLUM PORIFERA 



THE Porifera, or Sponges, are among the lowest of the 

 Metazoa. They are all fixed to the surface of a rock, or to 

 submerged timber or weeds, so as to be incapable of loco- 

 motion ; and have in most cases a general form and mode 

 of growth which suggest the vegetable rather than the ani- 

 mal kingdom. But, in essentials, as will presently become 

 evident, the Sponges are distinctly animal in character, and 

 the resemblances to plants are entirely superficial. 



The majority of Sponges, though none of them rise in 

 the essential features of their structure to a much higher 

 level than some of the colonies of Protozoa to which at- 

 tention has already been directed, are yet complicated and 

 difficult to understand, owing to their elaborate mode of 

 branching, and the fusion of the branches, and to the ex- 

 ceedingly intricate character of the skeletal parts. Some, 

 however, are free from these complications ; and it is in one 

 of these that the main characteristics of Sponges are best 

 studied. Such a simple Sponge is Sycon, a small Sponge 

 occurring attached to rocks on the sea-shore towards or 

 below low-water mark. Sycon gelatinosum has the form of 

 a tuft, one to three inches long, of branching cylinders 

 (Fig. 33), all connected together at the base, where it is 

 attached to the surface of a rock or other solid body sub- 



