LOWER ANIMALS 



19S 



//i\^ ASCON 



o E 









LEUCON or RHAGON 



Figure 1 2.2 Types of canal systems in sponges. 



bers of greater and greater complexity. In the sycon 

 type the pores are sunk into chambers that are in turn 

 parallel to the chambers where all the current- 

 producing cells are located. The leucon type displays 

 still greater ramification of chambers with further 

 withdrawal of the current-producing cells from the 

 central cavity. Most sponges are not solitary, but 

 occur as interconnected individuals (e.g., the com- 

 mon encrusting forms). 



The anatomy of the simplest sponge type is most 

 easily studied through the microscope. The outer 

 layer is composed of flattened cells. Within is a jelly- 

 like, nonliving layer containing living, wandering, 

 amoeboid cells, so-named because of their resemblance 

 to Amoeba, a protozoan. These amoeboid cells carry 

 on many of the life processes of the sponge. They 

 form spine-like, calcareous or siliceous spicules (spicule 

 composition and shape are used to characterize the 

 classes of sponges). Amoeboid cells also secrete horny 

 spongin, the material of the bath sponge and certain 

 other complex sponges. Both spicules and spongin 

 when present form a sponge's skeleton. The inner 

 layer of a simple sponge is principally current-pro- 

 ducing cells. Besides creating a water current, these 

 cells capture microscopic food organisms. The re- 

 sulting nutrients are probably distributed to the 

 sponge by amoeboid cells. An additional cell type, 

 the already mentioned pore cell, contains a canal 

 throughout its length. Pore cells are so oriented that 



the canal leads from the exterior to the central cavity, 

 forming the surface pores. Pore cells and these 

 current-producing cells are found in no other multi- 

 cellular animal. Current-producing cells, however, 

 exactly resemble certain free-living protozoans, some 

 flagellates. Because of this and the strange develop- 

 ment of sponges, some zoologists believe that sponges 

 had flagellate ancestors, but arose separately from 

 other multicellular animals. Although the micro- 

 scopic anatomy of sponges reveals contractile cells 

 around the mouth-like, large, external opening, 

 sponges lack any specialized nerve, rnuscle, or excre- 

 tory cells (Figure 12.3). 



Sponges are primarily marine animals found from 

 tide pools to the ocean deeps. They are extremely 

 varied in appearance and assume various flat, globu- 

 lar, and branched shapes. Most adults are attached 

 to living or nonliving objects in the sea; however, a 

 few species do not grow on objects. Those sponges 

 growing upon animals may, in a sense, be motile. For 

 example, some sponges grow around snail shells that 

 frequently contain hermit crabs rather than the living 

 snail; other species grow upon crabs. The only group 

 of nonmarine sponges is the strictly fresh-water 

 Family Spongillidae. Spongillidae grow as small to 

 fist-sized, irregular masses on plants or nonliving ob- 

 jects in streams, ponds, and lakes. Their basic colors 

 are yellow and brown, but some are green owing to 

 the presence of microscopic, chlorophyll-bearing 

 flagellates or green algae. 



Reproduction in sponges is both asexual and sexual 

 and generally different from that of other groups of 

 animals. Asexual reproduction occurs by budding 

 of individuals and gemmule formation. In budding, the 

 parent sponge remains an entity while a bulge in the 

 body wall grows to form a new individual. Gemmules 

 are formed in the jelly-like layer when clumps of 

 amoeboid cells are surrounded by a heavy, resistant 

 cyst wall. This latter type of reproduction is more 

 common and seemingly more valuable to fresh-water 

 sponges, because ponds may freeze in winter or dry 

 in summer and the resistant cysts make survival until 

 a favorable period possible. Sexual reproduction also 

 involves the amoeboid cells, which in different indi- 

 viduals give rise to a single egg or to many sperm. 

 The sperm are liberated and swim in the sea water. 

 If fertilization is completed, the sperm must first enter 

 through a pore cell of another sponge, then be en- 

 gulfed (like food) by the current-producing cells 

 which then become amoeboid, and finally be trans- 



