98 



COLLEGE ZOOLOGY 



low or deep water; for example, Microciona 

 in shallow water forms a thin encrustation 

 on rocks, while in deep water the colonies 

 become massive and reach a height of as 

 much as 6 inches. Some are branched like 

 trees, others are shaped like gloves, cups, or 

 domes. The majority are irregular and with- 

 out symmetry, although some are radially 

 symmetrical. Sponges vary in size from spe- 

 cies no larger than a pinhead to species that 

 are as big as barrels 8 feet in diameter. 

 Sponges are highly variable in color; some 

 are white or gray, and others are yellow, 

 orange, red, green, blue, purple, and velvety 

 black. 



Canal systems 



If it had not been for the development 

 of elaborate canal systems, sponges would 

 have remained in the simple condition of 

 Leucosolenia and would never have been 



able to become massive in size. The canal 

 system furnishes a highway for food through 

 the body and for transportation of excretory 

 matter out of the body. Three types are usu- 

 ally recognized (Fig. 48): (1) the simplest 

 or ascon type, as in Leucosolenia, (2) the 

 sycon type, as in Scypha, and (3) the 

 rhagon (leucon) type in which there are a 

 number of small chambers lined with 

 choanocytes. 



Skeletons 



The skeletons of sponges consist of car- 

 bonate of lime or silica (a mineral sub- 

 Stance akin to glass) in the form of spicules, 

 or of spongin in the form of fibers more or 

 less closely united (Fig. 49). Spongin is a 

 substance chemically related to human hair. 

 It is secreted by flask-shaped cells (spongo- 

 blasts). Spicules are deposited in cells 

 (scleroblasts, Fig. 49), and more than one 



Calcareous spicules 



Siliceous spicules 



Spongin with 

 spicules 



Spongin without 

 spicules 



Side view 



Thick wait 

 with spicules 



Germinal cells 



Development of 

 spicules 



Gemmules 



(asexual 



reproduction) 



Free swimming larva 



(amphiblastula of 



sexual reproduction) 



Figure 49. Parts of sponges. Spicules from various genera. Gemmules of a freshwater sponge. 

 (Amphiblastula after Parker and Haswell.) 



