ORDER i SILICISPONGIAE MONACTIXELLIDA 45 



tetra.xial spicnles an- very rarely forked or otherwise modified. Kach skeletal 

 (lenient behaves optically like a single ealeite crystal : axial canals are absent. 



The skeletal dements in spunk's are arranged chielly with reference to the 

 circulation of water tlmm-h 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 filires, or are packed in 

 b-t \\eeii the canals ; in still others they are united to form an irregular frame- 

 work, or may be soldered together in a regularly reticulated scaffold iii.u. 



The hornv til>res 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 tilled 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 : Myxospongiae, Ceratospongiae, 

 S&icispongiae, and Calcispongiae. The latter group stands in sharp contrast to 

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

 to- ether a group of equal value with the calcareous sponges. Skeletal elements 

 are absent in the Mi/xospongiae, 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 (Wiizocorallum), Jurassic, 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, 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. MONACTINBLLIDA. Zittel. 



(Monaxonia, F. E. Schulze.) 

 All skeletal elements uniaxial. 



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- 



