152 AN INTRODUCTION TO ZOOLOGY 



The second type (Fig. 75, B) is more complicated; the water flows 

 through the dermal pores (ostia) into the incurrent canals (inc.) ; 

 then through the chamber pores (prosopyles, pr. p.) into the radial 

 canals (fl. c.); from here it is propelled by the flagella of the 

 choanocytes into the gastral cavity (GC), finally passing out 

 through the osculum (osc.). In the third type (Fig. 75, C) there 

 are three distinct parts, (i) the water passes through the dermal 

 ostia (ost.) and by way of incurrent canals (inc.) reaches (2) a 

 number of small chambers (fl.c.) lined with choanocytes, thence it 

 is carried through (3) an excurrent system (exc.) to the gastral 

 cavity (GC), and finally out of the osculum. 



The skeletons of sponges are composed of spicules and spongin. 

 The former are secreted by cells of the dermal layer, and consist 

 of calcite and silica. The spongin is an organic substance. 



Reproduction. Reproduction is either asexual or sexual. 

 By the asexual method there are produced buds and gemmules. 

 Buds may be set free to take up a separate existence, or may 

 remain attached to the parent sponge, aiding in the formation of 

 a complex assemblage of individuals. Gemmules occur in the 

 fresh-water sponge, Spongilla, and several marine species. A 

 number of cells in the middle layer of the body wall gather into 

 a ball and become surrounded by protecting spicules. These 

 gemmules are formed in the autumn just before the death of the 

 adult sponge. In the spring they develop into new sponges. 

 They are of value in carrying the race through a period of adverse 

 conditions, such as the winter season. Sexual reproduction takes 

 place essentially as described for Sycon on page 149. 



Pieces c r sponges are capable of regenerating entire animals. 

 This characteristic enables sponge-growers to plant a bed of 

 sponges by scattering small pieces over the bottom of the sea in 

 favorable places. After a period of several years, animals of 

 commercial value may be gathered, and a new lot of " slips " 

 set out. 



Embryology. The principal stages of sponge embryology 

 may be presented briefly as follows: 



