50 COELENTERATA— PORIFERA phylum i 



(d) Anaxial or polyaxial bodies of spherical, cylindrical, stellate or 

 discoidal shape, which are not derivable from either of the three ground-forms, 

 occur in only a few varieties of recent and fossil siliceous sponges. 



Calcareous skeletal elements are much less complicated, and are genei'ally 

 smaller and more perishable than the siliceous. Their form is either triaxial 

 (triods), tetraxial (tetraxons\ or nail -shaped (inonaxons). The triaxial and 

 tetraxial spicules are very rarely forked or otherwise modified. Each skeletal 

 element behaves optically like a single calcite crystal ; axial canals are absent. 



The skeletal elements in sponges are arranged chiefly with reference to the 

 circulation of water through the canal-systems. In thin- walled forms they are 

 more or less closely crowded together, and are often regularly oriented in the 

 soft parts ; in other forms they are encased in horny fibres, or are packed in 

 between the canals ; in still others they are united to form an irregular frame- 

 work, or may be welded together in a regularly reticulated scaffolding. 



The horny fibres are totally destroyed during fossilisation ; calcareous 

 spicules are often wholly or partially dissolved, or are replaced by infiltrating 

 lime carbonate, and assume a dense fibrous appearance (Pharetrones). Like- 

 wise in siliceous sponges the skeletal elements are rarely preserved unaltered ; 

 as a rule the originally colloidal silica becomes crystalline, or is dissolved and 

 removed. The cavities thus formed may subsequently become filled with 

 infiltrating quartz, limonite or most commonly with lime carbonate. In 

 this manner the skeletons of fossil siliceous sponges are converted into calcite, 

 and, contrariwise, spicules that were originally calcareous may become silicified. 

 Hence the distinction between siliceous and calcareous sponges in the fossil 

 state depends entirely upon morphological characters, and not at all upon the 

 chemical composition of the preserved parts. 



Sponges are divided into four subclasses : Myxospongiae, Ceratospoiigiae, 

 Silicisponcjiae and Calcispongiae. The latter 'group stands in sharp contrast to 

 the other three, which are connected by intermediate forms, and constitute 

 together a 'group of equal value with the calcareous sponges. Skeletal 

 elements are absent in the Myxospongiae, whose bodies are composed entirely 

 of soft cellular tissues. The Cerafospongiae also lack imperishable hard parts, 

 the spongin fibres being entirely destroyed during fossilisation. The reputed 

 horny sponges from the Trias (BhizocoraUum), Jura and Cretaceous (Spongites, 

 Saxonicus, Paramudra, etc.) are either of inorganic nature or are zoologically 

 indeterminate. All fossil sponges, therefore, belong either to the Silicispongiae 

 or the Calcispongiae. The oldest forms are found in the Cambrian ; in the 

 Trias, Jura and Cretaceous they are very abundant. 



Subclass 3. SILICISPONGIAE. Siliceous sponges. 



Skeleton composed either exclusively of siliceous elements, or of horny fihres enclos- 

 ing siliceous spicules. 



Order 1. MONACTINELLIDA Zittel. 

 (Monaxonia F. E. Schulze.) 



All skeletal elements uniaxial. 



The Monacfiiu'llida include the majority of existing marine sponges, most 

 of which occur at moderate depths ; and also the few fresh-water forms 



