312 

 THE GEOUP SPONGI^E. 



The Turkey Bath Sponge as a Type Its Structure and Embryology Its Mode of Life Specific Distinction and Existing 

 Distribution Sponge-farming Forms and Colour of Sponges The Individuality Question Different Types of Canal 

 System The Three Primary Layers The Skeleton Spicule Forms Embryological Development Affinities of the 

 Sponges Their Classification General Characters of Existing Families Their Distribution in Space and Time. 



THE Sponges are a numerous, diverse, and yet compact, group of animals, manifesting, amidst a 

 remarkable diversity of minor characters, a fundamental similarity by which they are united closely 

 together, and separated from all the rest of the animal kingdom. In a word, they are Metazoa, 

 or multicellular animals, in which the endodermal layer characteristically consists, partly or wholly, 

 of flagellated collared cells. 



A clear idea of the nature of a Sponge will be most readily obtained from a description of a single 

 well-selected example, and none is better suited for the purpose than the common Bath Sponge. The 

 object which is usually denoted by that name is but the skeletal remains of the animal a delicate 

 elastic network, which so intimately pervades every part of the living organism that, after all the 

 other tissues are removed, it still presents a faithful model of the general form and structure of the 

 whole. 



There are several kinds of Bath Sponge, but the one to which we shall restrict our attention 

 is the fine Turkey Sponge (Euspongia officinalis), of which there are several well-marked varieties, 

 differing greatly in form. Some are cup-shaped masses, with thick walls, or more or less globular 

 clumps ; others flat, somewhat ear-shaped plates ; and others, again, encrusting patches from which 

 small tubes grow upwards. The colour of the exterior is usually some tint of brown, varying from 

 yellowish-grey to black ; within it is of a lighter shade, varying from greyish-yellow to colourless, 

 but in one variety it is rusty red. 



A thin skin covers the whole surface of the Sponge, rising, tent-like, about the projecting ends of 

 the chief fibres of the skeleton. These projecting ends can readily be seen with a lens on an unused 

 skeleton of a Bath Sponge. 



In various places, irregularly distributed, the skin is perforated by circular holes known as 

 oscula, which can be opened or closed by the movements of a delicate iris-like membrane which 

 forms their margin. The oscula are the terminal openings of wide tubes which descend into the 

 interior of the Sponge, repeatedly branching like the roots of a tree in their course till they become 

 too small to be followed by the unassisted eye. They are known as the excurrent canals ; 

 and the tubular spaces in the skeleton corresponding to them, as well as the general position of 

 the oscules, are clearly visible in the Sponge of domestic use. Besides the oscules, large circular 

 openings, characterised by the absence of the iris-like margin, are sometimes, but by no means always, 

 present. They lead into wide canals which are usually tenanted by some large marine worm (e.g., 

 Nereis costce), which was regarded by Peyssonel as "the essential animal and sole fabricant of the 

 Sponge, all the rest being merely a nidus or excretion ! " 



On examining the surface of the Sponge with a strong lens, there will be seen over those areas 

 devoid of oscules a number of thread-like ridges descending radiately down the tent-like elevations of 

 the skin, branching as they go, and united laterally by similar but transverse ridges into an irregular 

 network with polygonal meshes. A number of round apertures, called pores, are situated in these 

 meshes, and give them a sieve-like appearance (Fig. 2). The pores lead into a roomy space, 

 the subdermal cavity, which spreads beneath the skin ; from it canals descend direct into the 

 interior of the Sponge, and sooner or later become branched ; these are known as the incurrent 

 canals (Fig. 1). 



Thus the Sponge consists of a fleshy mass, supported by a network of elastic fibres, invested 

 with a skin, and traversed by two sets of canals excurrent canals, each opening by a single oscule 

 to the exterior, and incurrent canals, which communicate with the exterior by cribriform pore 

 areas. Nearly this much, if we except the distinction of the canals into two kinds, was well known 

 at a very early date, probably from the time of Aristotle, two thousand years ago ; but so little does 

 the structure, so far ascertained, resemble that of any other kind of animal, and so little light does it 

 throw on the real nature of the organism, that the earlier naturalists were unable to infer from it 



