SPONGES 



129 



151. Canal Systems. — In sponges there are three principal types of 

 canal systems, known as the ascon, sycon, and rhagon types (Fig. 51), of 

 which the ascon is the simplest. The body of a sponge of this type has 

 a thin wall which is penetrated by simple canals that run clear through 

 to the gastral cavity. In this type flagellated cells line this cavity. 



Oscula 



fr 



'Osculum 





Ostium 



Fig. 50. — Different types of sponges. A, Grardia ciliata (Fabricius), one of the Cal- 

 carea, a simple sponge showing colony formation and budding. X 2. B, skeleton of 

 Euplectella sp., a hexactinellid sponge known as Venus' flower-basket, showing the form and 

 general structure; the spicules are white and like spun glass. X ^. C, Chalina oculata 

 Pallas, one of the marine Demospongiae. (From Minchin, in Lankester's "A Treatise on 

 Zoology," by the courtesy of A. and C. Black.) X %. D, Ephydatia fluviatilis (Linnaeus), 

 a fresh-water sponge belonging to Demospongiae. {From Zacharias, "Die Tier- und Pflam- 

 enwelt des Siisswassers.") X /^. 



In the sycon type a more complex plan is presented, with incurrent 

 and radial canals. The ostia lead into incurrent canals which do not 

 open into the gastral cavity; radial canals open into the gastral cavity 

 but not to the outside. The two types of canals lie side by side and are 

 connected by minute pores. The radial canals are lined with the flagel- 

 lated cells. 



