6i 



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 P. E. Sohiilze. ee, Bctodei-mal 

 pavement epithelium ; gli, gastral cavity ; m, mesoderm ; p, pores ; gfc, ciliated cliambers ; zk, afl'er- 

 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 nepvous system is not yet with certainty proved to exist in the 

 Porifera. 



Reppoduction is either asexual or sexual. 



Asexual peproduction takes place by external or internal budding 

 or gemmation. 



Extepnal gemmation. — A sponge may put out buds at various 



Fig, CO.— Various forms of skeletal spiculaa 

 from Sponges. 



