64 



COMPARATIVE ANATOMY 



CHAP. 



Skeletons of silica and of horn fibres are found combined. The sili- 

 ceous or calcareous skeletons consist of small bodies of extraordinarily 

 different shapes, the so-called spicules, most probably formed in the 



FIG. 59. Part of a section through Halisarca lobularis, after F. E. Schulze. ec, "Ectodermal 

 pavement epithelium ; gh, gastral cavity ; m, mesoderm ; p, pores ; gk, ciliated chambers ; zk, affer- 

 ent canals ; o, eggs in different stages of segmentation. 



cells. There are uniradiate, triradiate, quadriradiate, sexiradiate, multi- 

 radiate forms, stars, spheres, etc. The skeleton of a sponge may con- 

 sist of only one sort of 

 spicule, or two or more 

 sorts may occur together. 



The single spicules lie 

 either loosely near each 

 other, or are cemented 

 together into coherent 

 frameworks. The same is 

 the case with horn fibres. 

 The ordinary bath sponge 

 is only a framework of such 

 horn fibres ; it is merely 

 the skeleton of a marine 

 animal (Fig. 56). 

 A nervous system is not yet with certainty proved to exist in the 

 Porifem. 



Reproduction is either asexual or sexual. 



Asexual reproduction takes place by external or internal budding 

 or gemmation. 



External gemmation. A sponge may put out buds at various 



FIG. 60. Various forms of skeletal spiculae 

 from Sponges. 



