ORDER I SILICISPONGIAE—MONACTINELLIDA 45 
tetraxial spicules are very rarely forked or otherwise modified. Each skeletal 
element behaves optically like a single calcite erystal ; 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 soldered 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). In 
silicious sponges also the skeletal elements are rarely preserved unaltered ; as 
a rule the originally colloidal silica becomes crystalline, or is dissolved and 
carried away. The cavities thus formed may subsequently become filled with 
infiltrating quartz, limonite, or most commonly with carbonate of lime. In 
this manner the skeletons of fossil silicious sponges are converted into calcite, 
and, contrariwise, spicules that were originally calcareous may become silicified. 
Hence the distinction between silicious 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 sub-classes :—Jyzxospongiae, Ceratospongiae, 
Silicispongiae, and Caleispongiae. 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 Myzxospongiae, whose bodies are composed entirely of soft 
cellular tissues. The Ceratospongiae also lack imperishable hard parts, the 
spongin fibres being entirely destroyed during fossilisation. The reputed horny 
sponges from the Trias (Rhizocorallum), Teese 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 Caleispongiae. The oldest forms are found in the Cambrian; in the 
Trias, Jurassic, and Cretaceous they are very abundant. 
Sub-Class 3. SILICISPONGIAE.  Silicious Sponges. 
Skeleton composed either exclusively of silicious elements, or of horny fibres enclos- 
ing silicious spicules. 
Order 1. MONACTINELLIDA. Zittel. 
(Monaxonia, F. E. Schulze.) 
All skeletal elements wuazial. 
The Monactinellida include the majority of existing marine sponges, most of 
which occur at moderate depths ; and also the few fresh-water forms (Spongilla) 
that are known. The skeleton, as a rule, is composed like that of the horny 
sponges, of anastomosing spongin fibres, which either encase rod-like spicules, 
or contain quantities of uniaxial silicious elements; sometimes the latter are 
also present in the soft parts. In each genus there are generally but one or 
but a few varieties of silicious elements present, which are uniformly dis- 
