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TEXTBOOK OF ZOOLOGY 



circumstances. In fresh-water forms these gemmules are formed in 

 the middle layer of cells; the parent individual then dies; and the 

 following spring new individuals emerge from the gemmules. 



Sexual Reproduction occurs here for the first time in our dis- 

 cussions. Sponges are usually hermaphroditic, but the germ cells 



Sexual I^eprodaction 



Fig. 55. — Methods of reproduction in Scypha. (Courtesy General Biological Supply 



House. ) 



Qranular cells 

 (Dermal epithelium) 



Segmentation cavity 



Ragellafced cells 

 {Qastral epithelium) 



Osculum breaks thru here 



-Dermal epithelium 



■ Gasbral epithelium 



Gastral cavity 

 (future cloaca) 



--Dloitopore 



Typical f ree-5W/mming 

 Awphiblastula 



Typical /Amphiblaifcu/a 

 at time of attachment 



Fig. 56. — Diagrammatic sections of Scypha larvae. 



of the male usually mature before those of the female. The repro- 

 ductive cells are produced in the jellylike middle layer. Fertiliza- 

 tion takes place here and cleavage division progresses. At about 

 the blastula stage the embryos are liberated through the wall of the 

 body as free-swimming, ciliated larvae. These later settle down, 

 become attached, and are modified to form adult, sessile sponges. 



