THE ANIMAL KINGDOM 385 



3. The germ cells are the reproductive cells, which in many 

 forms are segregated from the somatic cells early in ontogeny. 



4. Though asexual reproduction by fission or budding occurs, 

 there is always sexual reproduction from a fertilized egg or, less 

 commonly, an unfertilized egg. 



5. The developing egg undergoes cleavage; the cells or blas- 

 tomeres thus formed adhere to one another to produce a multi- 

 cellular complex. 



6. At least two germ layers develop : an ectoderm, forming the 

 external covering, and an endoderm, lining the alimentary canal 

 and its outgrowths. Between these, in the majority of meta- 

 zoans, a third germ layer, the mesoderm, is formed from which 

 muscles, vascular and other tissues and organs develop. 



The distinction between Metazoa and Protozoa is not sharp 

 since colonial protozoans, such as Volvox, consist of groups of 

 different kinds of cells, organically connected with one another. 

 Colonial Protozoa to a certain extent bridge the gap between 

 solitary Protozoa and Metazoa, but in the latter there is a greater 

 degree of interdependence among the cells of the individual 

 organism than there is between the individual members of a 

 protozoan colony. 



PHYLUM 2— PORIFERA 



Porifera (pore bearers) or sponges are lowly organized Metazoa 

 that do not seem to lie in the direct line of ancestry of the higher 

 forms. They are sessile, aquatic animals, most of which live in 

 the sea. The body of the simplest sponges (Ascon type) is 

 tubular in form and is attached at its closed basal end to the 

 substratum. The free end is provided with an opening, the 

 osmium. The thin walls are pierced with smaller openings 

 called pores (Fig. 217A). The principal cavity of the tube, the 

 gastral cavity, is lined with flagellated collar cells (Fig. 218), known 

 as choanocytes, which bear a striking resemblance to certain 

 flagellated protozoans such as Codosiga. The food of sponges 

 consists of small animals and plants and organic matter in the 

 water, which with its contents is drawn in through the pores by 

 the action of the flagella of choanocytes into the gastral cavity, 

 where the food is ingested and digested by the choanocytes, the 

 water leaving the gastral cavity by the osculum. In the Sycon 

 type of sponge the gastral cavity consists of numerous outpocket- 



