46 ZOOLOGY. 



be, with the outer world. These pores may open and then 

 be permanently closed, new ones opening elsewhere. The 

 osculum bursts open by the accumulation of water between 

 the two layers in the same manner as the pores. Finally, 

 in certain sponges the horny fibres grow out from the outer 

 cell-layer and extend inward, surrounding the spicules, the 

 latter developing from the middle cell-layer. 



It appears, also, that all sponge embryos form a two and 

 afterward three-layered sac (gastrula), in which in the sim- 

 plest sponges there is a primitive body-cavity and a prim- 

 itive month, while in the higher calcareous sponges and in 

 the silicious forms the body - cavity is only temporarily 

 open, being afterward filled up by the interior ciliated 

 cells, and thus forming a compact mass. 



In the sponges, also, the larva or free-swimming young 

 is a three-layered sac, which is either hollow or, more com- 

 monly, solid, and may attach itself at the end of jts free- 

 swimming life by one end to some fixed object. The body- 

 cavity may persistiu th'e simpler forms through life, though 

 in most sponges, there is no genuine digestive cavity, but a 

 large series of minute digestive sacs communicating by canals 

 with the large ones leading to the oscula. The more or less 

 regular spherical form of the young of mosi sponges becomes 

 lost as they grow ; they become irregular in form, encrust- 

 ing rocks, and their development retrogrades rather than 

 advances. 



In the fresh-water iS^ow^z7Za there is a special provision foi 

 the maintenance of the species. In autumn are formed the 

 so-called " seed," being capsules in which are enclosed eggs 

 which in the spring develop young sponges. This cyst or 

 capsule may be compared to the buds or winter eggs of the 

 Polyzoa or of the water-flea {Daphnia). ' 



From the members of the next branch, the sponges dififer 

 in the great irregularity of their form, the lack of a definite 

 digestive cavity and of tentacles. 



Order I. Calcispongim. — The sponges may conveniently 

 be divided into two orders. Those belonging to the first 

 secrete spicules of lime, and there are no digestive or ampul- 

 laceous sacs.but the minute canals are lined with ciliated cells. 



