326 NATURAL HISTORY. 



irregular crusts of beautiful colours ; sky-blue to violet, russet-red to flesh-colour, pale yellow, carmine, 

 and purple tints being common, though some are colourless. 



The Choiidrosiadce* are provided with a rough external rind of fibrous connective tissue, and 

 one of the two genera of which the family consists is furnished with stellate silicious spicules ; it 

 should therefore be placed in the Silicispongue, of which it, as well as the remaining genus, 

 which is devoid of hard parts, is probably a degraded descendant. 



CALCLSPONGIJE. 



These are a small but compact group, which has been closely studied by Haeckel, whose 

 brilliant monograph on the " Kalkschwamme," though marred by a vicious confusion of fact and 

 fancy, has both by its fancies and its facts given a more powerful impetus to the investigation oi 

 the whole group of Sponges than any work which has appeared since the time of Robert Grant. 



The skeleton here consists of calcareous spicules of various forms, acerate, tri-radiate, and 

 quadri-radiate, the tri-radiate being the most characteristic : they are never collected into fibres 

 nor united together by spongin, but occur separately immersed in the soft tissue of the Sponge, 

 so that after the death and dissolution of the organism they at once fall asunder, and being at 

 the same time very soluble in sea water are so quickly destroyed that it is very doubtful 

 whether they are capable of being preserved in the fossil state. Up to this time no fossil Sponge 

 unquestionably belonging to the Calcispongiae has been described. 



They are mostly very small Sponges, often of very regular geometric form; usually white, 

 though sometimes brilliantly coloured. 



The Asconest are simple sacs, with a completely flagellated endoderm; they may be single 

 (Fig. 8), or branched (Plate 71, Fig. F), or in other ways united into a common stock. The 

 Sy cones | are composite sacs, derived from the Ascones by a budding of Ascon-like sacs radiately from 

 the wall of a parent Ascon (Plate 71, Fig. G). It is a fact of great interest for the theory cf 

 development that a contimious or transitional series of species can be shown to exist between a simple 

 Ascon and a Sycon in which the radiate buds have all united together by their lateral surfaces to 

 form a complex tubulated wall (Plate 71, Figs. H, i) ; and especially that this most complex Sycon 

 passes through the various stages exhibited by these species in the course of its individual development. 



One of the common animals of the sea-shore is the little purse-shaped Sycandra (Granting 

 compressa, which occurs hanging mouth downwards from the under-surface of rocks, or their 

 attached seaweeds, between tide levels. Sections can easily be made of this to show the tubulated 

 structure of the wall, and by boiling it in caustic potash for a minute or two its beautiful calcareous 

 spicules can be freed from the soft tissues for examination under the microscope. The Leucones are 

 characterised by a complicated water canal system, which appears to belong to the same type as that of 

 Euspongia and the majority of the Sponges. The snow-white crusts of Leucandra nivea are by no 

 means rare on the under surface of between-tide rocks on the English coasts. 



The Calcispongice have a world-wide distribution, and are found from the sea-level down to a 

 depth of 342 fathoms. Of the 111 species described by Haeckel, nine are cosmopolitan, 68 are found 

 exclusively in the Atlantic, 12 in the Pacific and 22 in the Indian region. No doubt the greater 

 richness of the Atlantic region is due to its having been more thoroughly investigated than the others. 



SILICISPONGIJE. 



In this order, characterised by silicious spicules, the Sponges attain their fullest expression 

 and highest development. Its members are the most numerous, the most diverse, and some of 

 them the most complicated of the class. They are spread through all seas, at all depths, and 

 were already in existence in the early Cambrian times. The only family of Sponges (SpongUlina) 

 which inhabits fresh water belongs to them, and this inhabits the rivers of most existing continents. 



The Monaxonidae (usually known as the Monactinellidae, which is a misnomer, since it is the 

 one-axedness of their spicules, not their oiie-rayedness, which characterises them) are distinguished 

 by the presence of uni-axial, and the absence of tetractinellid and hexactinellid spicules. If quadri- 

 radiate spicules do occur in some genera they differ from those of the genuine Tetractinellida? in the 



* Greek, cfiondros, gristle. f Greek, asms, a wine-skin or leathern bottle. J Greek, sycon, a ftg. 



Greek, Icucos, white. 



