138 SPONGES 



spines, which are restricted to the termination of the shaft, and in some 

 cases assume the character of a grapnel (Proteleia, Acarnus). In the 

 interesting genus Trikentrion, however, the spicules which echinate the 

 skeletal fibres (see below) are branched at their inner end so as to have 

 two, three, or even four roots by which they are attached to the skeletal 

 fibre, and the branching here affects the axial thread, producing some- 

 times an imitation, as it were, of a tetraxon spicule (Fig. 90, h). 



(b) The microscleres, thougli usually monaxon, exhibit a wider range of 

 variation than is to be found amongst the megascleres, owing to their 

 being usually strongly curved or provided with prominent hooks or 

 spines. In this way arise certain constant forms, often of great system- 

 atic importance, such as the siyma (Fig. 48, a, 6, g), the toxa, the chela 

 (Fig. 48, /i), specially characteristic of the family Poeciloscleridae, and the 

 peculiar amphidiscs, developed in connection with the gemmules of some 

 Spongillinac (Fig. 56, amph). 



Of the polyaxon type, both streptasters and euasters are met with, 

 the latter form being, however, of rather exceptional occurrence. It is 

 extremely probable, moreover, that, with few exceptions, the streptaster, 

 when found in this group, represents a minute spined rhabdus, in which 

 the shaft has become shortened and the spines lengthened, and should 

 therefore be regarded as of the monaxon, rather than of the polyaxou 

 type. Spined rhabdi are of common occurrence as microscleres, and in 

 the Spoiujillinae they seem to be of caenogenetic origin and derived from 

 megascleres. The euaster would appear, in at least one family (Axinel- 

 lidae, to represent a further step in the reduction of a monaxon strept- 

 aster. In the other cases, where euasters occur (e.g. Tethyidae), the true 

 affinities of the sponges that possess them are shown by various secondary 

 characters to be with the Tetractinellida rather than with the typical 

 Monaxonida, and the spicules in question may in such forms be regarded 

 as primary euasters of the true polyaxon type, derived from a Tetraxonid 

 ancestor which has recently lost its tetraxon spicules. 



Union of the Spicules and their Arrangement in the Fibres. 

 Secondary siliceous deposits, for the purpose of uniting the spicules 

 into a framework, are unknown in this group, though in the 

 Spongillinae peculiar spicular systems of branching form, due to the 

 fusion of several independent monaxons, are of common occurrence 

 as an abnormality or variation which may become so frequent that 

 in some cases it must be considered as a normal feature of certain 

 species (Evans, 1899). 



Union between the spicules is effected either by means of 

 fibrous tissue or by spongin. A well-marked series of gradations can 

 be made out in this respect. In the most primitive types the 

 spicules are held together, if at all, by fibre cells. In the next 

 stage there are to be found amongst the fibre cells a certain number 

 of glandular cells (" spongoblasts "), derived from the external 

 epithelium (see above, p. 46), which become included m the 

 growing fibres and secrete spongin. Next the number of spongo- 



