i6o 



ZOOLOGY. 



terminate directly or indirectly in pores at the surface (whence 

 the name Porifera). In the more complicated sponges there 

 is such power of budding and lateral growth that there is 

 formed a dense tuft of sponge made up of many individuals 

 in organic connection with each other. In such sponges the 

 simplicity of the internal structure is lost, and the cloaca may 

 branch, opening to the exterior by a number of oscula. The 

 radial passages which penetrate the wall become much 

 branched and enlarged in special regions until the mesen- 

 chyma becomes honey-combed with the passages and cham- 

 bers. No animals are more profoundly influenced by their 

 environment, in the matter of the special form which the 

 individual assumes, than the sponges. Individuals which de- 

 velop in active currents differ much in bodily shape from 

 members of the same species which grow in sheltered places. 

 In all instances the form assumed appears to be correlated to 

 the external conditions. 



204. The Structure of the Body. In the typical condition 

 the body of a sponge consists of ectoderm and entoderm, 

 with a gelatinous mass between them in which are imbedded 

 cells of various kinds and spicules of hard material forming 

 a skeleton (Fig. 77, C). The ectQderm is usually of flattened 

 cells and covers the exterior. It lines the pores and the outer 

 ends of the passages by which the water passes to the interior. 

 The entoderm lines the cloaca and the radial tubes, and is 

 especially well developed in the pocket-like enlargements of 

 these tubes, when they occur (Figs. 76, ch\ 77, en 2 ). In these 

 passages the entoderm is more columnar in shape and is sup- 

 plied with flagella, by the action of which currents of water are 

 kept flowing inward to the central cavity. The middle mass or 

 mesenchyma, which lies between these two layers and makes 

 up the principal thickness of the body, consists x of numerous 

 cells of various kinds in a gelatinous intercellular substance 

 (Fig. 77, mes). Some of these cells are amoeboid or migra- 

 tory, others resemble the cells of connective tissue, others se- 



