82 



PORIFERA. 



The spicules of sponges in the diversity, symmetry, and intricacy 

 of their form, in the perfection and finish of their architecture, 

 constitute some of the most astonishing objects in natural history. 

 In view of them it is impossible to regard sponges as low in the 

 scale of evolution : such finish and such perfection of structure can 

 only have been reached as the result of a long process of evolutionary 

 changes. While it is pretty clear that the main function of the 

 skeletal structures is the support and protection of the sponge body, 

 it is by no means easy to gfve explanations of the diversity and 

 complexity of form which they present. The form of the megascleres 

 is probably connected with the form of the canal system, with which 

 they are in relation (F. E. Schulze) ; but the form and even the 



existence of the micro- 

 f scleres defies any reason- 

 able explanation. By some 

 spongologists the small 

 spicules are regarded as 

 functionless, and as having 

 on that account a greater 

 value for classificatory pur- 

 poses. However this may 

 be, all classes of skeletal 

 structures are utilised in 



classification, and although no single character is by itself of much 

 use for this purpose, the form of the microscleres is perhaps as 

 important as the form of the megascleres. 



It is therefore necessary to consider in some detail the forms of 

 sponge spicules. 



MEGASCLERES. 



There are five principal kinds of megascleres : (1) Monaxons rod-like megas- 

 cleres ; (2) Tetraxons megascleres with four axes, proceeding from a central 

 point, with four rays ; (3) Triaxons megascleres with three axes crossing at right 

 angles, with six rays ; (4) Polyaxons megascleres with several axes ; (5) Spheres 

 megascleres in which growth is concentric about the origin. 



Each of these classes contains many varieties, the most important of which 

 must be dealt with. 



I. Monaxons, megascleres in which growth is directed from a single origin in 

 one or both directions along a single axis. The ray or rays of a monaxon are 

 called actines. 



1. Khabdus, growth proceeds in both directions along the axis hence 

 diactine. The principal varieties of Rhabdus are as follows : 



With similar ends : 



a. Oxea needle-shaped, pointed at both ends (Fig. 72, 7). 



b. Tornote abruptly pointed at each end (8). 



FIG. 71. Calcareous spicules (triods) of Sycon. 



