86 BRITISH FOSSIL SPONGES. 



Class : SPONGLE. 



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. — Myxospongoe. 



Sponges without any skeleton, or with only a few scattered siliceous spicules 

 (v. Lendenfeld). 1 Unknown as fossil. 



Order II. — Ceratospongi^;. 



Sponges with skeletons of horny fibres, which may contain foreign bodies but 

 not proper spicules. Siliceous 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 being a siliceous monac- 

 tinellid, whilst the reputed horny Sponges, known as Spongites, are of an altogether 

 doubtful character. 



Order III. — Silicispongi^e. 



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. — Monactinellid^. 



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. 



