6o 



SYSTEMATIC ZOOLOGY 



dents, 

 which 



The colonies are squeezed to destroy the soft parts, 

 are then allowed to macerate, and the sponges are finally- 

 washed in fresh water, so that the skeleton is all 

 that remains. The best commercial sponges come 

 from the Mediterranean ; the finest quality is from 

 the Levant, the second best from the Adriatic, but 

 none grow on the coasts of Italy, France, Spain, or 

 the adjacent islands. Other sponges of not so fine 

 a quality are obtained in the Red Sea, among the 

 Bahamas, and off the coast of Florida. 



Fig. 43. Sycon 

 ciliatum. (After 

 Hyatt, from Par- 

 k'i and Has- 

 vvell*S Manual.) 



Order 4. Silicispongiae 



The Silicispongiae (Lat. silex, flint, and spongia, 

 a sponge) possess a skeleton of siliceous spicules 

 and often in addition spongin fibers. The spicules 

 are often very elaborately formed, and the entire 

 skeleton is sometimes extremely beautiful, as in the Venus' 

 flower-basket (Fig. 44), which lives in the deep sea. This is 

 a very large group of sponges, and they have a wide distribu- 

 tion ; here belong the common fresh-water species, many 

 which occur in the tide pools, and others at almost all depths of 

 the ocean. 



SUBTYPE II. CNIDARIA 



The Cnidaria ( Gr. KviSyj, a nettle) are of the same general type 

 or plan as the Porifera, --i.e. the body is a sac, whose wall 

 consists of ectoderm, entoderm, and a basement membrane or 

 mesenchyme. To this sac there is a single opening, which is 

 called the mouth, for all the food passes through it to the cosl- 

 enteron, since there are no pores or canals through the walls. 

 like those in the Porifera. The mouth, then, of the Cnidaria is 

 not analogous to the osculum of the Porifera. The characteristic 

 from which the Cnidaria derive their name is the presence of 

 certain modified ectodermal cells, known as nettle-cells (Fig. 43 ). 

 These cells possess a flask-shaped cavity, called a nematocyst, 

 which contains a long, delicate thread, coiled up, and when the 

 surface of the animal is irritated by some foreign body or sub- 

 stance, this thread is shot out violently, and thus serves as an 

 organ of defense against enemies and for capturing small animals 



