70 



ANIMAL BIOLOGY 



Sponges reproduce both asexually and sexually: asexually by 

 buds and by the liberation of groups of cells called gemmules 

 possessing the power to form new individuals; and sexually by 

 eggs and sperm. The zygote' develops into a tiny ciliated embryo 

 which after a short free-swimming existence settles down, becomes 

 permanently attached, and soon attains the adult structure. 



Osculum 



/* & & 



//Oscu-Vj 

 & lum i-» 



Gastral ^*-'} 



Prosopyl< £C Cavity 



»>" 



s/s t 



*> 



*ca^to 



9 



9. 



' Incurrent 

 ■=— canal 



"V^VA 



Apopyle 





Flagellated 

 chamber 



B 



Gastral cavity 

 Osculum. 



Flagellated 

 chambers 



Incurrent 

 canals 



Openings of excurrent canals 

 Dermal pores 



5^SubdermaI 

 ^Lcavity 



Excurrent canals 

 C 



Fig. 36. — Types of canal systems of Sponges. A, ascon type (Leucosolenia) ; 

 B, sycon type (Grantia); C, rhagon type (Spongilla, Euspongia). The arrows 

 indicate the direction of the current of water. The thick black line in A and B 

 represents the gastral layer; the dotted portion, the dermal layer. (From 

 Minchin, and Parker and Haswell.) 



The relationship of the Porifera to the series of Invertebrates 

 raises an interesting problem. The tissues of Sponges are, as we 

 have seen, not only too complex to be considered colonies of Pro- 

 tozoa, but they also are much less complex than those of the higher 

 multicellular animals. And they are organized in a radically differ- 

 ent way as well, including an apparent reversal of the two primary 



