62 COMPARATIVE ANATOMY CHAP. 



Order 5. Ceraspongia (Fig. 56). 



Skeleton consists of horn fibres (spongin). Proper spicules wanting. Fragments 

 of foreign spicules, sand, etc., are often used for strengthening. Spongelia, 

 Euspongia officinalis (bath sponge), Aplysina. 



The form of the body in the Porifem is so wonderfully varied that 

 no general description of it is possible, on account of their great variety 

 in shape. Sponges or sponge stocks can be knob-like, pear-shaped, 

 crust-like, funnel-shaped, cylindrical, or spherical. Many are irregularly 

 branched. Some have a radiate structure. All are attached, or have 

 part of their bodies buried in mud. 



With the exception of the Spongillidce, all sponges live in the sea. 

 In many sponges the external forms to be met with, even in one 

 and the same species, varies to an extraordinary degree. The same 

 individual even, in different parts of its body, may show differences 

 of texture and structure, and variations in the composition of the 

 skeleton. 



The inner structure of sponges is not less varied. As an example 

 let us take Olynthus (Fig. 57). This sponge is vasiform and rather 

 thin walled ; it is attached by its blind end, while 

 the opposite free end is broken through by an 

 opening (oseulum). The body wall is perforated 

 by pores which can open and shut. The water 

 streams through the pores into the body cavity, 

 which may be compared with the gastric cavity of 

 the gastrula, and flows out through the oseulum. 

 The wall, as far as is at present known, consists 

 of two layers : (1) an .outer layer formed of a 

 tolerably homogeneous fundamental substance, in 

 which are imbedded cells and calcareous needles ; 

 (2) an inner epithelium of collar cells. Perhaps 

 here also, outside the layer which contains the 

 skeleton, there is a thin tesselated epithelium, in 

 which case the body would consist of three layers 

 an outer ectodermal layer, an inner endodermal epithelium, and an 

 intermediate mesodermal layer of connective tissue. Those sponges, 

 which are essentially of the same degree of organisation as the Olynthus, 

 are called Ascones. 



A higher degree of organisation is attained when the body wall 

 becomes thicker and cylindrical tubes or pouches arranged close 

 together penetrate into the thickened wall radially round the central 

 cavity (Fig. 58). The outer surface of the sponge is then often raised 

 in numerous cones over these radial tubes. The radial tubes are lined 

 with collar epithelium, while the epithelium of the central or gastral 

 cavity is changed into a pavement epithelium. The outer pores 

 in this case lead first into the radial tubes, from these into the central 

 cavity, and thence through the oseulum to the exterior. Sycones. 



