SPONGE-SPICULES 



8 59 



...It' 



vals, giving them a jointed appearance. 1 The more recent authorities 

 on Sponges, such as Professor Hollas and Messrs. Ridley and Dendy, 

 have recognised that in the present state of our knowledge the spicules 

 which are ordinarily found in silicious Sponges belong to one of two 

 groups, which, as they differ considerably in size, may be called 

 megascleres (or, more correctly, megaloscleres) and microscleres. It is 

 to the definite arrangement of the former that, with or without the 

 addition of spongin, the sponge owes its definite skeleton ; the micro- 

 scleres give consistency to the 

 tissue of the sponge, and are ir- 

 regularly scattered throughout 

 its substance. If we desire to 

 give them physiological names 

 we may call the megaloscleres 

 skeletal spicules, and the micro- 

 scleres flesh-spicules. If we 

 bear in mind that in the 

 opinion of the most competent 

 spongiologists the polyaxial 

 spicules are the most primi- 

 tive, there is 110 practical 

 objection to our noticing them 

 in the reverse order, a method 

 which will be found to conduce 

 to simplicity of description. 

 In the examination of spicules. 

 it is necessary, first of all, to 

 distinguish between axes and 

 rays ; thus in the Monaxonida 

 the megaloscleres have but a 



at 



L..I 



FIG. 655. Structure of the chela of Mo- 

 naxonid Sponges: 1, tridentate anisochela 

 from in front; la, from the side; 2, la, 

 front and side views of a palmate isocliela ; 

 /, /', tubercle; at, at', anterior tooth or 

 palm ; U , It', lateral tooth or palm ; s, shaft ; 

 /, timbria. (After Ridley and Dendy.) 



single axis, but the growth 



from the point of origin may 



be on either side, when we 



have two-rayed or diactinal 



megaloscleres, or it may extend 



in one direction only, when the 



scleres are said to be monactinal. In the Calcispongise there are 



three axes and three rays : but in some sponges, such as Teiius's 



flower-basket, the growth is along both directions of the axes, so 



that while there are three axes there are six rays, or the spicules 



are hexactiiiellid. In others, such as Ceodia and the Lithistid 



Sponges, there are four axes, whence such forms are called tetraxonid. 



1 A minute account of the various forms of spicules contained in Sponges is given 

 by Mr. Bowerbank in his first memoir ' On the Anatomy and Physiology of the 

 Spongiada? ' in Phil. Trans. 1858, pp. 279-332; and in his Monograph of tlie 

 British S/m/ti/iaihr, published by the Kay Society. The Calcareous Sponges have 



, and Messrs. Ridley and Dendy, Report on the ' Challenger' Monax- 

 onida, pp. xv-xxi. 



