84 ANATOMY AND PHYSIOLOGY 



inhaled by the sponge, and transmit it to the mouths of the 

 incurrent canals which have their origin in the intermar- 

 ginal cavities. Enveloping the entire mass of the spoinge 

 we find the dermal membrane, in which are situated the 

 pores, for inhalation and imbibition of nutriment, and the 

 supply of the incurrent canals ; and the oscula, through 

 which the excrementitious matter and the exhausted streams 

 of water are poured from the terminations of the excurrent 

 canals. These parts are indispensably necessary, and are 

 always present in a living sponge. The attachment of the 

 Spongiadse to the body to which they adhere during life, 

 is effected by a basal membrane which presents a simple 

 adhesive surface, following the sinuosities of the body on 

 which it is based, entering into holes or cracks and filling 

 them up, thus securing a firm hold of the mass on which 

 they are fixed. When it so happens that the locality con- 

 sists of sand or mud, their bases frequently assume the 

 form of branching roots, which penetrate the mud or sand 

 to a considerable extent ; but they are never instrumental 

 to the nutrition of the animal — they are simply the anchors 

 by which it is fixed to its locality for life. 



We wiU now proceed to consider the structure and func- 

 tions of these organs in the order indicated at the beginning 

 of this volume, page 4 ; commencing with 



THE SKELETON. 



There are two important distinctive characters for con- 

 sideration in treating of the structure of the skeleton : — 

 1st, the material of which it is constructed; and, 2nd, the 

 mode of its arrangement. 



By selecting the material substance of the skeleton as 

 the means of dividing the Spongiadse into Orders, we obtain 

 three well-defined natural groups, which are again readily 

 divisible into Families, based on the mode of the arrange- 

 ment of the substance of which the skeleton is com- 

 posed. 



The first Order Calcarea has the primary essential mate- 



