122 Ouide to the Invertelrata. 



Sponges, position is at present altogether problematical, but tbey are placed in 



GALLEEY the same cases as the sponges until their true relations are known. 



X. 

 West Side. 



Wall-cases 2. — Calcispongi^. In this division the skeleton consists 

 7 & 8, principally of three- and four-rayed spicules, and also of simple, 

 cases" needle-like, uniaxial spicules, which are formed of carbonate of 

 11-16. lime fFig. 173^-w). There is the same manifold variety of form 

 in the Calcisponges as in Silicisponges, and their canal system is 

 of a similar character. 



The spicules in most of the recent forms are either loosely 

 distributed in the soft tissues, or have a definite radial arrangement 

 to form canals as in the Sycones, but in nearly all the known fossil 

 forms they are arranged in definite anastomosing fibres. 



The structure of Calcisponges in many instances has been very 

 greatly altered in fossilization. The spicules in the fibres have 

 lost their outlines, and the fibres now appear as if entirely formed 

 of granular or fibrous calcite. In other cases the fibres have been 

 replaced by silica, so that they remain after treatment with acid, 

 but all traces of spicules have been obliterated. In some specimens 

 of PJmretrospoyigia, preserved in solid flint, the outer portion of 

 the fibres has been replaced by silica, whilst their interiors yet 

 retain the original structure of carbonate of lime. The structure 

 even in the best-preserved specimens of Calcisponges is hardly at 

 all recognizable, unless in thin sections under the microscope. 



Fossil Calcisponges have been arranged in the following 

 families : — 



(a) Pharetrones. — In this division, which comprises the large 

 majority of the fossil forms, th© skeletal spicules are arranged side 

 by side in close contact with each other so as to form cylindrical 

 fibres, which anastomose together. The spicules are not organically 

 attached or fused together in any way, and it is surprising that 

 fibres formed in this way should have been preserved intact. In 

 some forms there is a relatively large three- or four-rayed spicule 

 in the axial portion of the fibre, which is enveloped by smaller 

 filiform spicules, whilst in others the spicules are equal or sub- 

 equal throughout. In many of the sponges there is a smooth or 

 wrinkled dermal layer formed of a close felt-work of spicules. The 

 Pharetrones have been divided into numerous genera. The most 

 important are Peronidella, Zitt., Corynella, Zitt., Masmostoma, 

 Promentel, Pharetrospongia, SoUas, and Solcospongia, Hinde. 



