52 



COELENTERATA— PORIFERA 



PHYLUM II 



Order 3. LITHISTIDA Schmidt. 



Massive, thick-walled, siliceous sponges, usually with complicated canal-system. 

 Skeleton composed of irregular tetraxons or monaxons (desmoms) which develop 

 nodose or root-like branches either at the extremities or all along the shaft, and are 

 firmly united hy zygosis. Symmetrical, tetraxial, uniaxial or polyaxial dermal and 

 flesh-spicules also present. 



The Lithistids are closely related to the Tetractinellids, and in the 

 opinion of many zoologists, should be embraced in the same order with them. 



The Lithistids are peculiarly well adapted for preservation, owing to the 

 massive stony character of their skeletons ; and their remains occasionally 

 form thick deposits, especially in the Jurassic and Cretaceous. Their outer 

 configuration is extremely variable ; most commonly it is crateriform, 

 cup-shaped, pyriform, globular, bulbous or plate-like ; while the body is 

 attached either by the base or by means of a peduncle. The canal-system 

 varies greatly in difterent genera, but is usually well developed and more or 

 less complicated. The four-rayed skeletal elements are interlocked by means 

 of the root-like branching ends of the rays, and the points of intersection 

 (nodes) with the ends of adjacent uniaxial spicules are thickened into balls. 

 The usually irregular uniaxial skeletal elements are interlaced on all sides by 

 means of root-like processes. Dermal and flesh-spicules are preserved only 

 under exceptionally favourable conditions, but are invariably present in 

 recent genera, and furnish valuable differential characters. The classification 

 of fossil Lithistids is based wholly upon the skeletal elements and canal- 

 systems. Five principal groups are recognised, whose subdivision into 

 families need not concern us at present : — Tetracladina, Eutaxicladina, Anomo- 

 cladina, Megamorina, and Rhizomorina. Existing Lithistids occur most abun- 

 dantly at depths ranging between 100 and 400 metres, but are occasionally 

 found as deep as 1800 metres. 



Suborder 1. TETRACLADINA Zittel. 



Skeletal elements composed of four usually equal rays, each of which encloses an 

 axial canal, and has extremities terminating in root-like strands or processes ; the 



spicules are intertwined 

 to form an open mesh- 

 work. Dermal spicules 

 either grapnel-like te- 

 traxons, frequently with 

 furcate prongs, or dis- 

 coidal with entire or 

 lohate margin ; or they 

 are nail - shaped or 

 cylindrical monaxons. 



The skeletal ele- 

 ments of the Tetra- 

 cladina are usually 

 symmetrical tetrax- 

 ons, whose four smooth, more rarely tuberculate or knotty rays intersect ap- 

 proximately at an angle of 109i". They occur in the Cambrian and Silurian, 



Fio. 48. 



Aulocopixim aii,ranU)nn OsviaXA. Diluvium; Sadovvitz, Silesia, a, Example 

 in 1/2 natural size ; //, Skeleton magnitled 00 diameters. 



