THE PHYLUM PORIFERA 



175 



57. The Classes of Sponges 



The arrangement of channels in the sponge provides for a convenient 

 structural classification, but this has not proved to be useful in sep- 

 arating the classes of the phylum. Instead, the classes are distinguished 

 on the basis of the skeleton present: (1) Calcarea, with a skeleton made 

 of calcium carbonate spicules; (2) Hexactinellida, with a skeleton made 

 of siliceous spicules, in which the basic spicule has six rays (Fig. 9.3); 

 and (3) Demospongia, with a skeleton made either of siliceous spicules 

 (never six-rayed), or spongin fibers, or both. 



Calcareous Sponges. The calcareous sponges are marine, shallow- 

 water forms of small size, including all of the asconoid and syconoid 

 and some leuconoid forms. The spicules have one, three or four rays 

 (Fig. 9.3, A, B and C). Spicules with three or four rays are interlaced 

 in the body wall, forming a relatively rigid framework. The one-rayed 

 spicules project from the body surface, especially around the osculum, 

 and serve to keep other organisms away. The choanocytes are consider- 

 ably larger than those of other sponges. 



Hexacf/ne//ic/ Sponges. Hexactinellid sponges are marine, deep- 

 water forms. Tlie six-rayed spicules are usually cemented together to 

 form rigid girders (Fig. 9.3, D and E). Since the skeleton remains in one 

 piece after the Hesh lias been removed, these glass sponges are often 

 used as decorations. Even in the living glass sponge the tissue is scanty. 

 Body structure is intermediate between syconoid and leuconoid, but 

 the epidermis is lacking. Euplectella (Fig. 9.1) has a large spongocoel, 

 and an osculum covered by a sieve plate that keeps out large objects. 

 Other glass sponges are Hattened, fan-shaped structures, one side of 

 which represents the spongocoel. These forms have no osculum but they 



Figure 9.3. Sponge spicules. A, Monaxon. B, Triaxon. C, Tetraxon. D, E, Hexaxons. 

 A, B and C, made of calcium carbonate, are found in the class Calcarea. The same shapes, 

 made of silicate, are found in the class Demospongia. Hexaxons, made of silicate, occur 

 in the class Hexactinellida. 



