Some of the Lower Metazoans 377 



ferentiated on the basis of the material from which these spicules are 

 formed. The skeleton may be formed by calcium, silica, or a softer 

 material known as spongin. 



The symmetry of the sponge is primarily radial although among 

 the larger species this symmetry is lost and they become asymmetrical. 



The pores in the walls allow the water to pass into the central cav- 

 ity or spongocoel. The water flows past the choanocytes and many of 

 the small animals and plants are captured by the flagella. Finally the 

 current emerges from a large opening, the osculum. 



Reproduction is either by sexual or asexual means. Asexually 

 they may reproduce by budding or the formation of special resistant in- 

 ternal buds known as gemmules. These latter are found more com- 

 monly among the fresh-water forms than in the marine ones. Re- 

 generation of broken parts can be so extensive that it must be considered 

 here as a method of asexual reproduction. 



During sexual reproduction, a single individual may be either mono- 

 ecious or dioecious. The ova and sperm apparently develop from the 

 archeocytes for there are no special ovaries or testes. The ova are fer- 

 tilized in the mesenchyme by sperm from another sponge and remain 

 in their position during a portion of the development. After the ovum 

 has developed into a peculiar blastula stage, it becomes flagellated and 

 the resulting larva escapes through the osculum. This larva swims about 

 for a short time before attaching to a solid object. 



The Arrangement of the Canal Systems. — The simplest type of 

 sponge structure is that illustrated by the saclike species, Leucosolenia. 

 This very simple sponge is found in shallow waters along coastal areas 

 and usually occurs as tubes with branches extending upward. These 

 branches may be considered as individuals, while the whole mass is a 

 colony. This species has the asconoid type of construction and may be 

 designated as an ascon. The body wall is penetrated by numerous in- 

 current pores or ostia which extend from the outer surface into the 

 spongocoel. These ostia are contained in individual cells. The spon- 

 gocoel is lined with choanocytes and the waving of the flagella propels 

 the water in through the ostia, past the choanocytes, and out the oscu- 

 lum which is located at the distal end. This simple type of construction 

 is actually found in very few species but it is the basic type from which 

 the others have developed. 



The next stage in the development of the canal systems is illustrated 

 by the members of the genus Scypha (formerly known as Grantia) (Fig. 



