630 THE BIOLOGY OF MARINE ANIMALS 



and echinoderms, and to a limited extent in certain molluscs, decapod 

 crustaceans, brachiopods, etc. The classification is arbitrary, and some 

 groups of invertebrates show much lability in skeletal organization. The 

 following sections are organized on a phyletic basis. 



Protozoa. Radiolarians possess a central capsule of organic material 

 together with an internal spicular skeleton (Fig. 14. 18). The latter frequently 

 takes the form of a lattice-work bearing projecting spines, e.g. Helio- 

 sphaera, Actinomma. Spicules are absent in Thalassicolla and Collozoum. 



Organic material in the skeleton of Protozoa is usually classified as 

 tectin, a variety of glycoprotein. The membrane of the central capsule of 

 Thalassicolla contains quinone-tanned protein {see p. 641). In most radio- 

 larians the internal lattice-work and spicular skeleton consist of silica. In 

 Acanthometra and other members of the Acantharia, however, the skeleton 

 is composed mainly of celestite (strontium sulphate). Values for SrS0 4 lie 

 around 65%; Si0 2 forms about 9% of shell, and calcium occurs in traces 

 (96). 



Porifera. All sponges, with the exception of the Myxospongiae, are 

 provided with an internal structural framework of mineral or organic 

 composition. It is usual to define the skeletons of the three classes of 

 Porifera as follows: Calcarea, skeletons consisting solely of calcareous 

 spicules ; Hexactinellida, sponges with a purely siliceous skeleton composed 

 of six-rayed (triaxonid) spicules; Demospongiae, skeleton composed of 

 siliceous spicules which are not triaxonid, or of spongin, or of an admixture 

 of both. 



The skeleton of sponges has supporting and defensive functions. In 

 Euplectella (Hexactinellida) the skeleton is a delicate cylindrical frame- 

 work of siliceous spicules consisting of longitudinal, circular and oblique 

 components. The arrangement is such as to offer maximal resistance to 

 torsion and compression, and combines lightness with high rigidity. 

 Myxospongiae, lacking a skeleton, are encrusting in habit. The mineral 

 matter of sponge skeleton is made up of spicules of definite and charac- 

 teristic shapes, amply described in taxonomic text-books. Analyses of the 

 skeletons of calcareous sponges give values for CaC0 3 of 71-85%; these 

 may be lower than true values because of impurities (Hircinia, Grantia, 

 Leucilla). There is some evidence that the calcareous sponges may contain 

 significant amounts of MgC0 3 , up to 7% in Leuconia (Table 15.1). 

 CaC0 3 exists as calcite in the spicules. In siliceous sponges, such as Venus' 

 flower-basket Euplectella, the skeleton is composed of nearly pure opaline 

 silica. Boron is a constant minor constituent of siliceous spicules (Suberites, 

 Tethya). 



Organic skeletons consist of fibres of a proteid substance, spongin, 

 arranged in a network, or in branching formation. In many Demospongiae 

 the siliceous spicules are bound together by spongin material, or incor- 

 porated in spongin. In the sub-class Keratosa the skeleton is composed 

 entirely of spongin fibres. This group includes the bath sponges of com- 

 merce (Spongia, Hippospongia). On the basis of X-ray analysis, spongin 



