PORIFERA. 221 



nectida and Dicyemida have a many-celled ectoderm, enclosing a solid mass 

 of cells in the Orthonectida, a single giant cell in the Dicyemida. Sali- 

 wella consists of a single layer of cells enclosing a central digestive space. 

 Since the Dicyemida live as parasites in the nephridia of cephalopoda, the 

 Orthonectida in worms and echinoderrns, it is possible that their low or- 

 ganization is the result of degeneration. 



METAZOA. 



Excluding the Protozoa, all the "branches of the animal kingdom 

 may be included under the head Metazoa, i.e. higher animals. 

 The point of union is that they consist of numerous distinct cells, 

 and that these cells are arranged in several layers. At least two 

 layers are present, a layer the ectoderm which bounds the body 

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

 Between these two a third layer can occur, which frequently is 

 separated by a body cavity into an outer or somatic layer forming 

 part of the body wall, and an inner or splanchnic 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 development of the 

 organization, which appears in varying grades in the specialization 

 of tissues and organs. In no metazoan is there lacking a true sexual 

 reproduction, that is one by sexual cells, but the fact must not be 

 overlooked that many species reproduce (possibly exclusively) by 

 unfertilized eggs in a parthenogenetic manner. Besides the sexual 

 reproduction many species, especially the lower worms and coelen- 

 terates, reproduce by budding and fission. 



For all the Metazoa the segmentation of the egg is characteristic 

 to a high degree. The fecundated egg divides into numerous 

 cells which, as segmentation cells (blastomeres), remain united and 

 form the germ. .N~o Protozoan has a true segmentation. Division 

 there produces new individuals which either separate completely or 

 exceptionally remain in slight connexion as a colony. 



PHYLUM II. PORIFERA (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 (recently subdivided into several subgenera). The ani- 

 mals have no powers of locomotion, but are attached to stones or 

 plants, along the shores or at depths up to 6000 metres (4000 



