SPONGES 37 



intercellular, in nature and formation. On the other liand, the general 

 development of the canal system precludes any homology between ostia 

 and oscula, and the great difficulty often found in distinguishing the two 

 sets of structures in some Demospongiae is clearly secondary. It should 

 be mentioned finally that in Euplecfclla and some of its allies parietal fjaps 

 are met with in the body wall, leading from the exterior into the gastral 

 cavity (Figs. 15 and 18). These openings have, however, nothing to do 

 with the canal system, and appear to be simply an architectural adapta- 

 tion to the animal's life-conditions. 



{h) Skeletal Sijstem. — A small number of sponges are entirely 

 witliout any supporting framework or skeletal structures. A few 

 others, mostly inhabitants of the deep sea, have, according to 

 Haeckel, a pseudaskeletoii composed entirely of foreign bodies, 

 without any elements secreted by the sponge itself ; the true nature 

 of the organisms in question is, however, very doubtful. 



The vast majority of sponges, howevei', possess a true skeleton 

 (autoskeleton) composed of elements secreted hy the sponge itself 

 {autophya, Haeckel), which may be supplemented to a greater or 

 less extent by admixture of foreign particles {xenophya, Haeckel), 

 such as sand grains, skeletons of minute organisms, or spicules of 

 other sponges, taken up by the sponge from its surroundings. The 

 autoskeleton is always a secretion of the cells of the dermal layer, 

 and takes the form either of mineral sclerites or spicules, or of an 

 organic substance termed spongin, occurring usually either as a 

 cementing substance, or as fibres. The spicules may be composed 

 either of carbonate of lime in the form of calcite, or of colloid 

 silica (opal), with in each case a slight admixture of organic matter. 



a. Spicules. — The morphological variations of the. sponge spicule 

 are very numerous, and their classificatory importance necessitates 

 a complete and systematic nomenclature of the principal types of 

 form. Each spicule, of whatever material composed, is typically 

 made up of a greater or less number of rays or arms, representing 

 directions of growth, which radiate from the centre of the spicule, 

 ie. from the starting-point of the secretion, and are laid down 

 along a number of ideal axes. Theoretically, the number of rays in 

 a spicule will be either equal to, or double, the number of axes. 

 In point of fact, however, the number of rays actually present 

 may be far less than the number ideally possible for any given 

 type of spicule, either as the result of a secondary reduction of 

 spicule rays primitively present, or it may be, by persistence of the 

 spicule in a still more primitive condition in which the full number 

 of rays has not yet been acquired. Thus a spicule with three 

 morphological axes has typically six rays, but the number of the 

 latter may be reduced to two or three or even to a single one. 



The number of axes which can be recognised in a given 

 type of spicule is expressed by adjectives terminating in " axon," 



