ZB.— THE PORIFEES. 



237. THE SPONGES. 



General oonsideeations.— The natural group of Sponges with which the public have 

 become familiar, through their constant use of the domestic varieties, remained until comparatively- 

 recent times in the much disputed ground between the animal and vegetable kingdoms. The 

 careful investigations of several distinguished modern naturalists, howe.ver, finally established 

 their animal nature beyond all question, and they were accordingly transferred to the kingdom in 

 which they properly belong. But some years were yet to pass before theit true affinities with 

 other animals could be definitely determined, and they were grouped provisionally with the so- 

 called Protozoa, a somewhat heterogeneous assemblage of such low forms as did not agree struc- 

 turally with any of the four great branches or subkingdoms, then recognized as composing the 

 animal kingdom. Still later researches have clearly proved that the Sponges have a much higher 

 organization than the Protozoa proper, and propagate by means of eggs, while the members of the 

 latter group do not. Compared as a whole with all the groups above the Protozoa, the Sponges 

 appear to be the lowest in structure, and, moreover, they stand apart by themselves as a distinct 

 group, which, in the more receni; division of the animal kingdom by most authorities into some 

 seven subkingdoms or branches, in place of the original four, assumes the rank of a subking- 

 dom called Porifera, and comeg into the plan of classification between the Protozoa and the 

 Gcelenterara. 



The term Sponge conveys to the minds of most people simply the idea of an irregular, soft, 

 flexible mass of open structure, whose exterior is generally much roughened by projecting points, 

 and pierced by numerous holes, leading toward the interior, and whose structure, more closely 

 examined, is seen to consist of a fine net-work of small, horny fibers. This typical Sponge, as we 

 may term it, from its being the form most commonly known and observed, is the ordinary Sponge 

 of commerce, which, though limited in (its range to but a few tropical and subtropical regions, is 

 collected in great quantities and sent to all parts of the world. The commercial Sponges, which 

 are the only ones of economical importance to mankind, all belong to a single natural genus, 

 Spongia, and-form, so far as the number of species are concerned, but a small portion of the entire 

 branch Porifera. 



As stated above, the general conception of a Sponge is a fibrous skeleton, and nearly all 



Sponges possess a skeleton or the rudiments of one, but this varies greatly in character in the 



different divisions of the branch. In the commercial Sponges and their allies, the skeleton is 



horny and more or less flexible, consisting of fine fibers interwoven and joined together. In 



another group, the skeleton is composed of horny fibers intermingled with which are many 



siliceous spicules, causing it to have a much stiffer and harsher structure. In a third group, the 



so-called siliceous Sponges, the skeleton is entirely made up of siliceous spicules, which may be 



scattered singly through the soft substance of the Sponge, or joined together in bundles. These 



spicules vary in shape, some being simple and , straight,* and others pyramidal, star-shape, or 



granular. A fourth group, the calcareous Sponges, has a skeleton of calcareous materials, disposed 



in lines oi- columns at right angles to the walls. The recent members of this group have the 



843 



