THE SPONGES 35 



The beautiful Venus' s Flower Basket (Euplectella) and the 

 strange-looking Glass Rope (Hyalonema) Sponges offer a contrast 

 in structure to the Bath Sponge, their skeletons consisting 

 throughout of siliceous spicules bound together by a siliceous 

 cement. In the Slime Sponges (Myxospongida) l no skeleton is 

 formed. 



The minute spicules of lime or silica vary greatly in shape in 

 different species, forming most beautiful objects for the micro- 

 scope, and as they are of very great importance in the study and 

 classification of the Sponges, a systematic and highly technical 

 nomenclature of the principal types of form has been established. 2 



The Sponges are grouped under three divisions, each founded 

 on the character and composition of the skeleton. The three 

 classes are : (i) the Calcarea, all of which are characterised by 

 a skeleton in which the material is calcareous ; (2) the Hex- 

 actinellida, 3 characterised by the possession of six-rayed or 

 hexactinal spicules ; 4 and (3) the Demospongiae, 5 in which the 

 skeleton may be composed of siliceous spicules of various types 

 (never triaxon), or of spongin occurring either alone or in com- 

 bination with siliceous spicules or foreign bodies, or the skeleton 

 may be altogether absent, as in the Slime Sponges. Although this 

 third division comprises such a vast assemblage of different forms, 

 yet the most divergent are linked together by a complete and 

 gradual series of intermediate forms. 



The Sponges included in the class Calcarea mostly frequent 

 shallow water, and grow most luxuriantly in shady, sheltered 



alities. They form a comparatively small group (some 

 200 recent species), all characterised by a skeleton composed of 

 spicules of carbonate of lime, the spicules being three-rayed, 

 four-rayed, and needle-shaped. Examples of Sponges belonging to 

 this class are the simple vase-shaped Ascon, Leucosolenia, and 

 the Sycon already described. 



The Hexactinellida include many very beautiful and inter- 

 esting Sponges, nearly all inhabiting great ocean depths, ranging 



1 Muxa, slime. 



8 For a systematic description of the Spicules, see Vol. I. Cambridge Nat. Hist. ; 

 and Part II. of Lankester's " Treatise on Zoology." 



* Hex, six; aktis, ray. 



* Known as a triaxon type of spicule : tri, three ; axon, axis. 



* Demes, multitude. 



