174 ''WE ANIMAL KINGDOM 



Ascoiioid sponges have a single large chamber, the spongocoel, 

 lined with choanocytes. The incurrent pores and osculum lead directly 

 to and from this chamber. Incurrent pores develop as holes through 

 tube-shaped cells, the porocytes. 1 liese cells develop from amebocytes, 

 and may degenerate, leaving simple, small holes in the body wall. 



The body wall of syconoid sponges resembles a folded version of 

 the asconoid wall. At least some syconoid sponges actually develop from 

 asconoid-like juvenile forms. During development the body wall pushes 

 out to form numerous finger-like projections, carrying the choanocytes 

 in their internal cavities. Where the outer sides of the projections 

 touch, they usually fuse. The arrangement is such that any four pro- 

 jections will enclose a space, the incurrent canal. The former incurrent 

 pores are now internal, and are called prosopyles. Whether or not the 

 prosopyles are formed in porocytes is debatable, but if they are, the 

 porocytes soon disappear, for all prosopyles in the adult are simple 

 holes in the body wall. At the outer surface of the body new incurrent 

 pores open into the incurrent canals. The cavity of each finger-like 

 projection is the radial canal, which opens into the spongocoel by an 

 internal pore. All of the choanocytes retreat into the radial canals, and 

 a simple epidermis develops as the lining of the spongocoel. The ex- 

 current pore is an osculum similar to the asconoid osculum. 



The leuconoid type represents a further folding of the wall. The 

 gastrodermis pushes out from the radial canals into the body wall to 

 form a series of spherical flagellated chambers. Each chamber has a 

 single inlet from the incurrent canal, the prosopyle, and a single outlet 

 to the radial canal, the apopyle. Incurrent pores, incurrent canals and 

 radial canals remain as in the syconoid type. The increased bulk given 

 the body wall by this additional folding results in a shrinkage of the 

 original main cavity. In leuconoid sponges the spongocoel is divided 

 into confluent excurrent channels leading to the osculum. All of the 

 choanocytes are in the flagellated chambers, and the radial canals are 

 lined with epidermis. The fresh-water sponges and most of the marine 

 sponges are leuconoid. 



Although the leuconoid structure is understandable as a modifica- 

 tion of the syconoid type, it should be noted that many of the leuconoid 

 sponges develop directly to the leuconoid tyj^e without passing through 

 asconoid or syconoid stages. 



The efficiency of the sponge as a pump is related to its structural 

 plan. The only source of power is the beating of the choanocyte flagella, 

 which is not coordinated in any one chamber. Choanocytes surrounding 

 the incurrent pores or prosopyles propel water toward the interior, and 

 the water escapes through the osculum. In the asconoid sponges the 

 flagellated chamber is large and the force produced by the flagella is 

 directed toward the middle, so that flow out of the osculum is passive. 

 In the leuconoid type the flagellated chambers are small and the cho- 

 anocytes are located so that they propel water toward the excurrent 

 opening. Thus they not only draw water in through the prosopyles but 

 also actively direct it outward to the osculum. 



