MONAXONIDA 2\J 



Order I. Halichondrina. 



We have already seen typical examples of the Halichondrina 

 in Halicliondria panicea and Ejphydatia Jiuviatilis. Within the 

 Halichondrina the development of spongin reaches its maximum 

 among spiculiferous sponges, and accordingly the Ceratosa take 

 their multiple origin here (p. 220). Among Halichondrina 

 spongin co-operates with spicules to form a skeleton in various ways, 

 but always so as to leave some spicules bare or free in the flesh. It 

 may bind the spicules end to end in delicate networks (as in 

 Reniera or Gellius), or into strands, sometimes reaching a con- 

 siderable thickness (as in Chalina and others). In a few cases 

 there appears to be a kind of division of labour between the 

 spicules and spongin, the latter forming the bulk of the fibre, i.e. 

 fulfilling the functions of support, while the spicules merely beset 

 its surface as defensive organs, rendering the sponge unfit for 

 food. Fibres formed on this pattern are called plumose, and are 

 typical of Axinellidae. The distinctive fibre of the Ectyoninae 

 is as it were a combination of the Axinellid and Chalinine 

 types : a horny fibre both cored with spicules and beset with 

 them. Spicules besetting the surface of a fibre are termed 

 " echinating." Whenever its origin has been investigated, 

 spongin has proved to be the product of secretion of cells ; in 

 the great majority of cases it is poured out at the surface of the 

 cell, and Evans showed, 1 at any rate in one species of Spongilla, 

 that the spongin fibres are continuous with a delicate cuticle at 

 the surface of the sponge. In Reniera spp. occurs a curious case 

 of formation of spongin as an intracellular secretion. A number 

 of spherical cells each secrete within themselves a short length of 

 fibre ; they then place themselves in rows, so orientated that their 

 contained rods lie end to end in one line. The rods then fuse 

 and make up continuous threads ; the cells diminish in breadth, 

 ultimately leaving the fibre free. 2 



Order II. Spintharophora. 



These corticate forms are further characterised by the arrange- 

 ment of their megascleres, which is usually, like that of most 



1 Quart. J. Micr. Sci. xli. 1901, p. 477. 

 2 Loisel, J. de VAnat. et Phys. xxxiv. 1898, p. 1. 



