64 LOWER INVERTEBRATES. 



Oeder III. — KERATOIDEA. 



These are the true horny sponges, and from an economic point of view, are the 

 only ones which have any jjractical value. The skeleton consists of fibres of sj)onge- 

 horn or keratode, forming a network in the mesoderm. They are littoral forms not 

 usually found in water more than seventy-five fathoms in depth. They genei-ally avoid 

 sandy or muddy localities, preferring rocky ground or coral reefs. Passing by the 

 genus Darwinella, for which a sub-order has been formed, we come to the sponges of 

 commerce. 



Sub-Order I. — Sponging. 



The SponghiEe are characterized by having the fibres of the skeleton solid, but in 

 places where the water is filled with floating matter, they usually have a core of 

 foreign material, a fact which we have previously mentioned. 



The marketable kinds are all of one genus, Spongia, that from which all the 

 sponges derive their common name. There are only six species with, however, 

 numerous varieties, which are offered for sale, and in fact these may be reduced to 

 three sjjecies, if one so chooses. Three of the species are from the Mediterranean 

 and the Red Sea, and three from the Bahamas and Florida. Other species of this 

 genus have a very general distribution, but they are all confined to the equatorial 

 and temperate zones within an area on either side of the equator which is limited 

 by the isotherm or average temperature for January of 50° F. The Spongia grani- 

 inea and Spongia cerrebriforrnis are occasionally used in Florida and Bermuda, but 

 are not exported. 



The marketable sponges owe their excellence to the closeness, fineness, and resili- 

 ency of the interwoven fibres of the skeleton. The Mediterranean appears to be 

 jsartioularly favorable to the j)roduction of specimens with skeletons possessing these 

 desirable qualities in the greatest perfection. Those from the Red Sea are next in 

 rank, while those of our own shores, though corresponding species to species with these 

 and the Mediterranean forms, are coarser and less durable. Thus Spongia equina, 

 the Horse or Bath Sponge of the Mediterranean, is finer than the Spongia gossypina, 

 the Wool Sponge of Florida and Nassau, though it otherwise resembles it closely. 

 Spongia zimocca, the Zimocca Sponge, represents in the Mediterranean waters the 

 much coarser Spongia corlosia and Spongia dura, the Yellow Sponge and Hard-head, 

 on the American side. SjMngia adriatica, the Turkey Cup-Sponge and Levant 

 Toilette-Spoiige of the Mediterranean, answers to the finest though not the best of our 

 sponges, Spongia tubulifera. 



It is probable that the Red Sea and the Mediterranean were both colonized by 

 sponges from the Caribbean Sea, and, strictly speaking, the six marketable species 

 ought to be classed as three species with six principal varieties, differing from each 

 other according to their habitat. This conclusion is borne out by the facts that the 

 Caribbean Sea contains more species of this genus than any other locality, that no 

 marketable sponges are found in the Indian or Pacific oceans, and that the differences 

 in quality cited above are occasioned in these and other sponges with fibrous skeletons 

 by any change from shallower to deeper water, or from water loaded with sediment to 

 clearer waters. In each of these cases a finer sponge is the result, and this correlates 

 directly with the fact that even in the Mediterranean the marketable kinds are found 



