9,9-) 



PORIFERA. 



fathoms) . They form sjoherioal masses,, thin crusts, small cylinders, 

 or upright branching forms. Frequently the shape varies so that 

 one cannot speak of a typical form. It was also difficult to decide 

 about the animal nature of the sponges. Striking movements of 

 the body are rare; only by aid of the microscope can one see 

 motion — the opening and closing of the pores and the streaming 

 of tlie gastrovascular system. 



The simplest sponges, the Ascons (fig. 161), are thin-walled 

 sacs, fixed at one end, and with an opening, the osculum (func- 

 tional anus), at the other. The cavity of the sac, the ' stomach,' is 

 a wide digestive cavity into which water bearing food obtains 

 entrance through numerous small openings or pores in the body 

 wall. The basis of the body is a homogeneous or fibrous connective 



Fig. 161. 



Flfi. 162. 



Fig. IdX.—Olynthus. (After Haeokel.) e, spicules; j, eggs; o, osoulnm; p, pores; «, 



stomach.* 

 Fig. 163.— Section of wall of Sucnndra raphnnus. (After Schulze.) e, ectodermal 



epitbelium; en, collared flagellate cells; m, mesoderm witli connective-tissue 



cells; o, eggs; st, calcareous spicules. 



tissue permeated with branching cells (fig. 1G2) covered externally 

 by a thin layer of pavement epithelium which is easily destroyed. 

 This epithelium (earlier called ectoderm) and the connective 

 tissue (mesoderm) are now regarded as a common layer, ' meso- 

 ectoderm,' since it has been shown that the pavement epithelium 

 is often genetically only connective-tissue cells which have spread 

 over the surface. On the other hand there is a distinctly differen- 

 tiated entoderm in the shape of a one-layered flagellate epithelium 

 lining the stomach, the cells of which (en) recall the Choanoflagellata 



