222 



PORIFERA. 



fathoms) . They form spherical 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 the 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 s 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 



en. 



ek.. 



FIG. 161. 



Fio. 162. 



FIG. 161. Olynthus. (After Haeckel.) e, spicules; z, eggs; o, osculum; p, pores; tt,, 



'stomach.' 

 FIG. 162. Section of wall of Sycmidra raphanus. (After Schulze.) e, ectodermal 



epithelium; e?i, collared flagellate cells; m, mesoderm with connective-tissue 



cells; o, eggs; s, calcareous spicules. 



tissue permeated with branching cells (fig. 162) 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 Choanofl agellata 



