26 SPONGES 



and difficult to preserve, being easily dissolved out. In Calcarea 

 the cells of the dermal layer, and more especially the flat epithelium 

 and the porocytes, contain numerous opaque granules, which are 

 the seat of the pigment in coloured forms. When the sponge is 

 placed in alcohol, the colouring-matter dissolves rapidly out of the 

 granules, making the specimen a dull white or brownish colour, and 

 in fact reducing it to the condition of the forms without pigment. 

 In many Demospongiae, on the other hand, the pigment is more 

 resistant. Aplysina aerophoba is remarkable for possessing a pale 

 yellow pigment which becomes blue, and finally black, on exposure 

 to air, apparently by oxidation (Krukenberg). In alcohol it turns 

 reddish-brown. 



(d) Consistence, etc. Different sponges yield very different sensations 

 to the touch, according to the degree to which the skeleton is developed, 

 the nature of the materials composing it, or the texture of the surface of 

 the skin. The Myxospongida are soft, slimy, and easily squashed. The 

 more primitive Ascons, for example Clathrina clathrus (Fig. 7), are 

 excessively delicate when fully expanded, and collapse by their own weight 

 if lifted out of the water, but acquire considerable firmness and rigidity as 

 the result of contraction. Many calcareous and siliceous sponges, on the 

 other hand, have the surface roughened by projecting spicules, while the 

 body may be brittle or friable and easily broken, or it may be very tough 

 and even of almost stony hardness. In the Keratosa, the body is yielding 

 and slimy to the feel, but, at the same time, excessively tenacious, very 

 difficult to tear or pull apart. This feature is due to the tough elastic 

 spongin fibres composing the skeleton, and is found also in Monaxonida 

 according to the degree to which spongin is developed as a constituent of 

 their supporting framework. 



Many sponges have, when living healthily, a strong and disagreeable 

 odour, rather resembling garlic. This characteristic is very pronounced 

 in the common fresh-water sponge. 



2. Anatomy and Histology. 



The Olynthus. 



The Organisation of Sponges in General. 



(a) Canal System. 



(b) Skeletal System. 



(c) Histology. 



The Olynthus. The simplest known type of sponge, in structure, 

 as well as in form, occurs, as has been said, as a transitory stage, 

 the so-called Olynthus, 1 in the life-history of all calcareous sponges. 

 In the Olynthus the problems of sponge anatomy and physiology are 

 reduced to their lowest terms, and all sponges- may be regarded 

 ideally as derived from it, even though the Olynthus stage may not 

 actually appear in their ontogeny. 



1 The organism iu question received its name from Haeckel, who was under the 

 impression that it represented an adult generic type. 



