REPORT ON THE MONAXONIDA. XV 



proper understanding of the various forms which they assume. For further details the 

 reader is referred to the works cited below. ^ 



The spicules of the Monaxonida may be very conveniently divided into two classes. 

 Those of the first class constitute (with or without the aid of horn-like cementing 

 substance, or spongin) the true skeleton of the sponge. They commonly occur associated 

 together in tracts or in definite fibres, and are usually, when full grown, much larger 

 than those of the second class ; consec^uently the name megasdera is applied to them. 



The spicules of the second class occur, as a rule, irregularly scattered through the 

 soft tissues without taking part in the formation of the true, continuous skeleton. Their 

 function is in most cases very doubtful. Owing to their minute size they are termed 

 microsclera. 



Megasclera. 



In the Monaxonida the megasclera are (with few exceptions), as the name of the group 

 implies, uniaxial ; that is to say, the imaginary axis of the spicule, which finds its 

 nearest visible representative in the axial thread, remains unbranched, although it may 

 be more or less curved, or even bent at rather a sharp angle. Such a spicule may either 

 grow in two directions from its point of origin, or in one direction only. In the former 

 case two equivalent and usually similar rays arise more or less diametrically opposite to 

 one another, and the spicule is consequently said to be diactinal, the point of origin 

 remaining at or near the centre of the spicule. In the latter case one ray only is formed, 

 and the point of origin, commonly represented by a l^ulbous enlargement of the axial 

 thread, remains at the end of the spicule, which may or may not be swollen ; in this case 

 the spicule is said to be monactinal. Although it is impossible in many cases to 

 demonstrate with certainty the manner in which the growth of a spicule takes place, yet 

 the division into monactinal and diactinal is a very convenient one for all practical 

 purposes. The diactinal appears to be the simpler type, from which the monactinal is 

 derived by abortion of one of the rays. 



A. Diactinal megasclera. 



Here we may distinguish four fundamental types, which, however, run into one 

 another by imperceptible gradations, but for which it is convenient to have distinct 

 names for purposes of description. 



1. Oxea (woodcut, Fig. I., 1); these are straight or curved spicules, more or less 

 fusiform, and gradually pointed at each end. 



1 Gf. Bowerbank, Mon. Brit. Spong., vol. i. p. 6, &c.; Carter, Ann. and May. Nat. Hist., ser. 2, vol. xx, p. 23, pi. i.; 

 and ser. 4, vol. xiv. p. 100, pi. x.; Vosmaer, Bronn's Klassen u. Ordnungen d. Tliierreiclis, Porifera, pp. 187, 436, &c. 



