PORIFERA 201 



METAZOA. 



Excluding the Protozoa, all the phyla of the animal kingdom are 

 included under the Metazoa, i.e., higher animals. The point of union is 

 that they consist of numerous distinct cells, arranged in several layers. 

 At least two layers are present; one — the ectoderm — bounding the 

 body externally, and a second — the entoderm — lining the digestive tract. 

 Between these two a third may occur, frecjuently separated by a body 

 cavity into an outer or somatic layer forming part of the body wall, and an 

 inner or splmidinic layer forming part of the intestinal wall. This middle 

 layer is called mesoderm, no matter whether there be a body cavity or not. 



The multicellular condition allows a higher organization, which ap- 

 pears in varying grades in the specialization of tissues and organs. No 

 metazoan lacks a true sexual reproduction, that is one by sexual cells, but 

 the possibility must not be overlooked that some species may have lost 

 fertilization and may reproduce exclusively by unfertilized eggs in a par- 

 thenogenetic manner. Many species, especially the lower worms and 

 ccelenterates, also reproduce by budding and fission. 



The segmentation of the egg is characteristic of all Metazoa. The 

 fecundated egg divides into numerous cells which, as l:)lastomeres, remain 

 united and form the germ. No Protozoan has a true segmentation, 

 division producing new individuals which either separate completely or 

 remain in slight connection as a colony. 



PHYLUM II. PORIFEEA (SPONGIDA). 



The Porifera, or sponges, the most familiar representative of which is 

 the bath sponge {Euspongia officinalis), are, with few exceptions, marine. 

 In fresh water occur but a few species of Spongilla. The animals have 

 no powers of locomotion, but are attached to stones or plants, along the 

 shores or at depths up to 4000 fathoms. They form spherical masses, 

 thin crusts, small cylinders, or upright branching forms. Frequently the 

 shape varies so that there is no typical form. Striking motions are rare; 

 only with the microscope can one see the opening and closing of the pores 

 and the currents of the gastrovascular system. 



The simplest sponges, the Ascons (fig. 159), are thin-walled sacs, 

 fixed at one end, and with an opening, the osciduni (functional anus), 

 at the other. The cavity of the sac, the 'stomach,' is a wide digestive 

 cavity into which water bearing, food enters through numerous small 

 pores in the body wall. The basis of the body is a connective tissue 

 permeated with branching cells (fig. 160) covered externally by a thin 



