II 



PORIFERA 



63 



In most sponges, however, the " canal system " is more complicated. 

 The collar epithelium is limited to numerous so-called "ciliated 

 chambers," which are sac-like, and generally lie 

 scattered in the much thickened mesoderm of 

 the body wall (Fig. 59, gk). The pores of the 

 outer surfaces of the body lead into much- 

 branched canals of varying width, which are 

 lined with tesselated epithelium; these, as 

 afferent canals, enter the 'ciliated chambers. 

 Other canals of varying width, which often 

 unite into larger canals, lead out of the 

 chambers as efferent canals into the variously 

 shaped central cavity, which again opens out- 

 ward by means of an osculum. Such forms 

 are known as Leucones. 



The movement of the flagella of the collar 

 epithelium maintains a constant stream of water 

 through the canal system of the sponges. The 

 water enters by the pores, passes through the 

 canal system, and flows out again through the 

 osculum. 



The canal system may vary extraordinarily in details. 

 Its structure and arrangement are of importance in 

 classification. 



The coalescence of the afferent canals often causes a 

 system of large lacunae and cavities lying quite near the 

 surface, the sub-dermal spaces ; into these the pores open 

 either directly or through canals, the water passing on 

 from the sub-dermal spaces by special canals into the 

 ciliated chambers. 



The canal system of the sponges may be greatly de- 

 veloped in comparison with the solid matter of the 

 middle layer (mesoderm), or the solid tissue may pre- 

 ponderate. In the first case the sponge has a loose, in 

 the second a firm, texture. 



p 1G . 53. Sycandra ciliata, 

 Haeckel, after Vosmaer. 

 Longitudinal section through 



the bod y wal1 in tlie upper 



The mesoderm of the sponges is represented 

 by a middle layer of connective tissue, chiefly 

 gelatinous, with cells imbedded in it. The 

 latter are either spindle-shaped or star-shaped, occasionally vacuolated. 

 Some of them often contain colouring matter (pigment cells) ; others 

 can move like Amwbce (migratory cells). Long spindle-shaped and 

 finely-branched cells occasionally lie concentrically at the commence- 

 ment of the afferent canals, and no doubt serve as contractile cells 

 for closing the pores. 



The mesodermal connective tissue is, in sponges, the place of for- 

 mation of the very varied skeletal structures. These consist either 

 of carbonate of lime, or of silica, or of horn known as spongin. 



