ZB. THE POKIFERS. 



237. THE SPONGES. 



GENERAL CONSIDKK \TIONS. The natunil group of SjKjnges with which the public have 

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

 recent limes in ilie iniieli disputed ground between the animal and vegetable kingdoms. The 

 careful investigations of several distinguished modern naturalists, however, finally established 

 fl.eir animal nature he ond all question, and they were accordingly transferred to the kingdom in 

 \\Iii-.-Ii tliey properly jelong. Hut some years were yet to pass before their true atlinilies with 

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

 called rft<>:,,,i. :\ somewhat heterogeneous assemblage of such low forms as did not agree stiue 

 tin-ally with any of the four great branches or subkingdoms, then rccogni/cd as composing the 

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

 organi/ation 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 Pn>tiw>n, the Sponges 

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

 group, which, in the more recent division of the animal kingdom by most authoiities into some 

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

 dom called Porifera, and comes into the plan of classification between the Pruttwm and the 

 Ccelentera rt< . 



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

 llexible mass of open structure, whose exterior is generally much roughened by projecting |xiints. 



and pierced by numerous holes, leading toward the interior, and whose .structure, re 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. Tue commercial S|Mngcs. which 

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

 s'/wH///(f. and form, so far as the number of species are concerned, but a small portion of the en tin- 

 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 

 di 1Vercnt. divisions of the branch. In the commercial Sponges and their allies, the skeleton is 

 horny and more or less flexible, consisting of tine tibcrs interwoven and joined together. In 

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

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

 so-called siliceous Sponges, the skeleton is entirely made up of siliceous spiciiles, which may In- 

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

 ^picnics vary in shape, some being simple and straight, and others pyramidal, star shape, or 

 granular. A fourth group, the calcare.nis Sponges has a skeleton of calcareous materials. <li>p.i-. il 



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



843 



