SPONGE-SPICULES 



859 



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

 on Sponges, such as Professor Sollas arid 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 no 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 

 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 Calcispongiae there are 

 three axes and three rays ; but in some sponges, such as Venus'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 hexactinellid. In others, such as Geodia 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 

 Spongiadae' in Phil. Trans. 1858, pp. 279-332; and in his Monograph of the 

 British Spongiadce, published by the Kay Society. The Calcareous Sponges have 

 been made by Professor Haeckel the subject of an elaborate monograph, Die Kalk- 

 schwdmme, Berlin, 1872. For enumerations and classifications of the various kinds 

 of spicules, see Professor Sollas, art. ' Sponges,' in the 9th edition of the Encycl. 

 Britannica, and Messrs. Kidley and Dendy, Beport on the ' Challenger' Monnx- 

 <ni id a, pp. xv-xxi. 



-at 



L..I 

 2 



FIG. 655. Structure of the chelae of Mo- 

 naxonid Sponges: 1, tridentate anisochela 

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

 front and side views of a palmate isochela ; 

 t, t', tubercle ; at, at', anterior tooth or 

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

 /', fimbria. (After Eidley and Dendy.) 



