SKELETAL STRUCTURES. Go 



together in different ways, so as either to form a continuous framework or loose 

 open fibres, which serve as a support to the soft fleshy portions of the Sponge. 

 These mineral particles are termed spicules, a word used in a general sense to 

 indicate the individual element or particle of the skeleton, whatever may be its 

 form. The spicules of fossil Sponges, like those of recent ones, are composed 

 either of silica or of carbonate of lime. In all essential features they resemble the 

 spicules of existing Sponges, and it may be assumed that like these latter they have 

 been formed in cells in the mesoderm of the Sponge. Spicules consist of an outer 

 wall of concentric layers, enclosing in most, if not in all cases, a delicate longitu- 

 dinal axial canal, which extends either partially or entirely the length of the spicule, 

 and into each of its main arms or rays. This axial canal is usually much wider in 

 fossil spicules than in recent forms ; in the living condition the canal appears to 

 have been filled with protoplasm, but in the fossil state it is usually occupied by a 

 mineral substance of the same general character as the spicular wall. If the in- 

 filling material has the same optical characters as the wall of the spicule, the canal 

 can no longer be distinguished, and the spicule appears solid throughout ; but not 

 unfrequently the infilling substance of the canal is more resistent than the spicular 

 wall, and remains even when this latter has been dissolved away. Between each 

 of the concentric layers of the spicular wall there appears to have been originally 

 a thin film of organic substance. The structural layers are but rarely seen in 

 fossil spicules; they can, however, be occasionally distinguished in the rod-like 

 spicules of Uyalostelia (PL VI, figs. 2 a, 2 c, 3/). 



Two different kinds of spicules are usually recognised in recent Sponges, 

 skeletal-spicules and flesh-spicules, and both kinds are present in the fossil 

 forms, though the latter are of very subordinate importance in the fossil state, 

 owing to their rare occurrence. The skeletal-spicules form the main body of the 

 Sponge and are usually of much larger dimensions than the flesh-spicules. These 

 latter, on the other hand, exhibit a much greater variety of form than the skeletal- 

 spicules ; they are not attached together, but merely held in position by the soft 

 tissue of the Sponge, and consequently are not often found in their natural position 

 in fossil Sponges. 



The two chief considerations in the study of fossil Sponges are, first, the 

 character of the individual spicules, and next, the mode in which they are combined 

 to form the skeleton. The skeletal-spicules of fossil Sponges have been arranged 

 in groups based on the number and disposition of their axes or rays. Taking 

 first those of siliceous Sponges, the following typical divisions may be enumerated : 



1. Monactinellid spicules, or those in which a single axis is present. 



2. Tetractinellid spicules, in which there are four rays or axes, which radiate 

 from a common centre in the same direction as the axes of a regular, three-sided 

 pyramid. 



