PORIFERA 169 



passages which penetrate the wall become much branched and 

 enlarged in special regions until the mesenchyma becomes 

 honey-combed with the passages and chambers. No animals 

 are more profoundly influenced by their environment, in 

 respect to the special form which the individual or colony 

 assumes, than the sponges. Individuals which develop 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 

 varied external conditions. 



209. The Structure of the Body. — In the typical condition 

 the sponge consists of an outer epithelium (dermal layer) and 

 its derivatives, an inner epithelium (gastral layer), and an un- 

 organized middle region (mesenchyma). From certain unusual 

 occurrences in the early development of sponges we are not sure 

 that the dermal and gastral layers in them are homologous with 

 the ectoderm and endoderm in the animal kingdom generally. 



The outer epithelium is usually of flattened cells. These 

 cover the whole outer surface and line the incurrent canals. 

 From this layer arise certain specialized cells which come to 

 lie deeper and even to invade the mesenchyma. In the dermal 

 layer are cells that secrete the hard parts, as spicules of lime 

 and spongin fibres. In the middle region are amoeboid cells, — 

 which ingest, store, and convey foods, — and reproductive cells. 

 The inner epithelium, or gastral layer, lines the general cavity 

 and the tubes and chambers which penetrate the body wall. 

 In the cloaca the gastral layer is flattened; but in the radial 

 canals it is columnar or flask-shaped, collared, and flagellate 

 (Fig. 79, D). These cells by means of their flagella create the 

 inward currents of water that bring food to the animal. 



In the mature specimen all these tissues are penetrated 

 and supported by the spicules or threads of secreted skeleton. 

 These may be calcareous, siliceous, or homy. The sponge of 

 commerce illustrates the last. The spicules may be isolated 

 and independent as in Grantia, or become fused into a continu- 

 ous framework. But for this framework the otherwise soft 

 animal would collapse into a shapeless mass and thus close the 

 openings whereby water brings the oxygen and food. It is the 



