PRINCIPAL GROUPS OF THE SPONGES. l6l 



Class II. Calcispoxgle. 



Family i. Ascoxes (Ascetta, Szc.) 



,, 2. Leucoxes {Leucaudra, &c.) 



„ 3. Sycoxes (Grantid). 



„ 4. Pharetroxes (Coryne/Zci, Szc.) 



Class I. Plethospoxgle. 



The Sponges included in this class are occasionally destitute of 

 hard structures, but the great majority possess a skeleton, which may 

 be composed of horny fibres alone, or of siliceous spicules alone, or 

 which is formed by a combination of these two sets of structures. 

 In no case is the skeleton composed of carbonate of lime. 



Order i. Myxospoxgle. — The Sponges of this order are entirely 

 destitute of skeletal structures, or, at most, possess a few scattered 

 siliceous spicules. The type-genus is Halisarca, comprising a num- 

 ber of soft fleshy sponges, widely distributed in recent seas, which 

 form crusts upon submarine objects. No example of this order, as 

 might be expected from the soft nature of the organism, is known as 

 occurring in the fossil condition. 



Order 2. Ceratospoxgle. — In this group of Sponges, the skele- 

 ton is composed of the horny substance known as " spongin,*' and 

 there are either no proper spicules, or in some cases a few siliceous 

 spicules may be scattered in the mesoderm. The horny fibre of the 

 skeleton forms a close reticulation or network (fig. 48, a), and can 

 be shown to consist of a delicate axial thread of organic matter sur- 

 rounded by a laminated horny sheath. In many cases, the horny 

 fibre includes numerous sand-grains or other foreign bodies in its 

 interior (fig. 48, b), these being taken in at the free-growing ends of 

 the fibres, to which they form a kind of core, replacing the soft or- 

 ganic axis which in some cases is alone present. The type of this 

 order is the recent genus Euspongia, comprising the Turkey Sponges 

 of commerce. No undoubted fossil representatives of the Cerato- 

 spongice have been as yet detected in the fossil condition, the cylin- 

 drical antler-shaped bodies from the Cretaceous rocks of Germany, 

 which Geinitz described under the name of Spojigites, being of an 

 altogether problematical nature. 



Order 3. Moxactixellid^e. — This order comprises an extensive 

 series of Sponges characterised by the possession of a skeleton which 

 is typically composed of horny fibres with included spicules of flint. 

 The spicules vary much in form, but are always uniaxial, being most 

 commonly fusiform, pin-shaped, or bow-shaped (fig. 49, a, b, c, and 

 fig. 52). In recent Monactinellids the proportion borne by the spi- 

 cules to the horny fibre is very variable, and in some types the skele- 

 ton consists almost wholly of uniaxial spicules without any, or with 



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