58 



SYSTEMATIC ZOOLOGY 



living on the shells of crabs ; the crab walks about, and thus the 

 sponge acquires a new supply of water and food, while the plant- 

 like appearance of the sponge 

 protects the crab from its ene- 

 mies. The association is, how- 

 ever, not very intimate, and is 

 perhaps more properly called 

 commensalism. Numerous fos- 

 sil sponges have been found, 

 but they are all members of 

 groups still in existence ; they 

 are most abundant in the chalk 

 formations. 



The sponges may all be in- 

 cluded under one class, bearing 

 the same name as the subtype ; 

 and this class may be divided 

 into four orders, based on dif- 

 ferences in the skeleton. 



FIG. 42. Sycandra raphanus l a calcareous 

 sponge; larva in the blastula stage; seen in 

 optical section; enlarged. (Alter F. E. 

 Si hulze, from 1 latschek's I -ehi buch.) 



CLASS PORIFERA 

 Order 1. Myxospongiae 



The Myxospongiae (Gr. ixv^a, mucus, and airoyyid, a sponge) 

 possess no skeleton whatever. They are found in the sea, 

 incrusting stones and seaweeds, and are often brightly colored. 



Order 2. Calcispongiae 



The Calcispongiae (Lat. calx, lime, and spongia, a sponge) 

 have a skeleton consisting of spicules of calcium carbonate; the 

 spicules are of various forms, some needle-shaped, others tri- 

 radiate or quadriradiate. These sponges are exclusively marine 

 and are found in water of no great depth. They frequently 

 live singly and often form small colonies a few centimeters in 

 diameter ( Figs. 43 and 40). They are usually not brightly col- 

 ored, but of an inconspicuous gray or buff. 



Order 3. Ceratospongiae 



The Ceratospongiae (Gr. fcepa<;, horn, and airoyyid, a sponge) 

 are so called because the skeleton has the appearance of horny 



