482 HYATT'S REVISIOX OF THE 



Sub-order SPONGINiE. 



Family Spoxgiad^. 



The .skeleton of this group differs from that of the ApljsinoS; as pointed out by Bower- 

 bank and Schmidt, in the absence of the large tube occupying the axis of the fibres. The 

 fibre is generally spoken of as solid, but in reality has a minute channel in the interior, 

 which sometimes seems to be absent in dried specimens. This is probably due to the con- 

 traction of the walls of the fibre, for normally there exists in all fibres and spicules, at some 

 stage of their growth, a hollow axis. The aspect of the keratose material is also similar to 

 what it is in the Aplj^sino?, consisting of concentric laj'ers, and the intimate structure of 

 these layers or concentric coats does not differ essentially. They are made up of fine 

 fibrillae, which may be observed at the broken ends of fibres in most species, whereas in the 

 Aplysinaj, these concentric coats appear to be composed of more homogeneous keratode, 

 and the fibrillas are distinguishable only in the coarser skeletons. The fibrillce are contin- 

 uous, and not at all similar in aspect to the curious bulbous headed threads observed in 

 some species of the Hircinia^. 



There seems to be a direct connection between the dermal membrane and the vertical or 

 radiatory fibi-es, and in some cases also between this part and the secondarj- or connecting 

 fibres. This is unquestionably also the case in the genus Tuba, and other true Chalina 

 forms. 



In Spongia proper, the dermal membrane is loaded with foreign matter in some localities, 

 and these find their way into the fibres very rapidly. The commencement of a fibre is 

 trumpet-shaped, the keratose matter gradually shading off" into the soft membranous dermal 

 coat of the exterior of the sponge.^ As the whole mass increases in size, this introversion of 

 the dermal membrane carries into the centre of the fibre a string of broken siliceous 

 spicules and grains of sand. The secondary or connecting fibres are not usually, except in 

 Hircinia camjjana and Dijskica fragilis, loaded with any foreign material. This, and the 

 frequent presence of aborted twigs, seems to indicate that they are formed by a process of 

 budding from the primary fibres. 



The same mode of formation is evident also in Aplysina and Tuba, but the dermal mem- 

 brane appears to be harder and clearer of foreign matter. The keratose skeleton is not 

 therefore, as is generally claimed, wholly the product of the mesoderm or sarcodous body 

 of the mass, but partly an ectodermal product, and bearing in most cases the marks of its 

 derivation, not only in the presence of foreign material, but in the continuously tubular 

 structure of even the most solid looking fibres. The concentric coats of keratode are divi- 

 sible into two kinds, those which are produced primarily by the growth of the extremity, 

 which forms at one time several concentric coats, and those which are produced or deposited 

 around these subsequently^, and are evidently secreted by the mesoderm. 



The range of form is quite as great as it is in the Aplysina?. The mass of the forms in 

 Spongia are solid and rounded, varying to the cup-shaped or fistular ; none even of the fis- 



^ This is also shown by Barrois to be the case in the Dt-;- gatliered into bundles and enveloped by keratode, irhich is 

 macidon fruticosa. In the young of this species the spicules formed by a trumpet-shaped introveifion of the ectoderm 

 are shown in the figure and described by the author as projecting into the mesoderm. Ann. des Sc. Nat., June, 1876- 



