l68 ZOOLOGY 



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 

 external conditions. 



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

 the sponge consists of an outer epithelium and its derivatives 

 (ectoderm), an inner epithelium (entoderm), and an unorganized 

 middle region (mesenchyma). From certain unusual occur- 

 rences in the early development of sponges we are not sure 

 that the ectoderm and entoderm in them are homologous with 

 those layers 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. Among these 

 are fibrous contractile cells, and cells that secrete the hard parts, 

 as spicules of lime and spongin fibres. In the middle region 

 are also amoeboid cells, which ingest, store, and convey foods, 

 and reproductive cells. The inner epithelium, or entoderm, 

 lines the general cavity and the tubes and chambers which 

 penetrate the body wall. In the cloaca the entoderm is flat- 

 tened; but in the radial canals it is columnar or flask-shaped, 

 collared, and flagellate (Fig. 79,!)). 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 horny. 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 



