86 BRITISH FOSSIL SPONGES. 
Cuass: SPONGLA. 
Definition.—Bodies of very variable form, consisting principally of a soft fleshy 
mass, enclosed by an epithelium of a single layer of cells. The body is penetrated 
by a system of canals opening into chambers, and communicating with the exterior 
by numerous smaller apertures or pores, and larger or vents. In the majority of 
forms the body is supported by a skeleton, which is composed either of horny 
fibres or of siliceous or calcareous spicules. 
Order I.—Myxosponci2. 
Sponges without any skeleton, or with only a few scattered siliceous spicules 
(v. Lendenfeld)." Unknown as fossil. 
Order IJ.—Crrarosponcia. 
Sponges with skeletons of horny fibres, which may contain foreign bodies but 
not proper spicules. Suiliceous spicules rarely present, scattered in the mesodermal 
tissues (v. Lendenfeld). 
No Sponges of this group are definitely known as fossils, the form described 
as such by Carter under the name of Dysidea antiqua beg a siliceous monac- 
tinellid, whilst the reputed horny Sponges, known as Spongites, are of an altogether 
doubtful character. 
Order [I1.—Sruicisponcaim. 
Sponges with skeletons of siliceous spicules, either held in position by the 
soft portions of the Sponge, or united together in various ways to form skeletal 
fibres. 
Sub-Order 1.—MonactiNnELLIDa”. 
The skeletal-spicules possess a simple unbranched axis. They may be enclosed 
in distinct fibres or merely held together by spongin. The usual forms are 
acerate, acuate, cylindrical, or conical. Flesh-spicules of various forms may or may 
not be present. 
Owing to the mode in which the spicules are held together by the fleshy or 
horny parts of the organism, entire Sponges, or even fragments, are of very rare 
1 «Monogr. Austr. Sponges,” ‘ Proc. Linn. Soc. New South Wales,’ vol. ix, 1884, p. 339. 
