724 Mr. Edward A. Minchin [May 6, 



sufficiently well known as a mineral, and forming also the skeleton of 

 many forms of animal life. In the latter condition it can be well 

 studied in the very simple group of organisms known as Ascons, the 

 most primitive order of calcareous sponges. 



In Ascons, as in other calcareous sponges, the skeleton is made up 

 of minute splinters or spicules of calcite, which always conform to one 

 of three types of form ; (1) rod-like or needle-shaped spicules, usually 

 more or less curved, and always with unlike ends ; (2) three-rayed or 

 triradiate spicules, having each three rays meeting at a central point ; 

 and (3) four-rayed or quadriradiate spicules, consisting each of a 

 basal system of three rays, exactly similar to the triradiate spicules, 

 and an additional or fourth ray tacked on to it. The three basal rays 

 may therefore be termed the triradiate system in the three-rayed and 

 four-rayed spicules alike, irrespective of the presence or absence of 

 the fourth ray. 



With regard to the triradiate systems, it may further be noted 

 that three classes can be distinguished amongst them. Sometimes the 

 three rays are unequal in size, and irregular in arrangement, making 

 a figure which is quite asymmetrical ; such forms are, however, com- 

 paratively rare. More usually the triradiate systems exhibit a definite 

 symmetry which follows one of two patterns. In the first place, the 

 rays may meet at equal angles, so that, irrespective of the unequal 

 development of the rays themselves, the spicule is symmetrical about 

 three planes. In the second place, the angles may be such that the 

 spicule shows a marked bilateral symmetry, having an unpaired 

 and two paired angles, with corresponding unpaired and paired rays. 

 Thus irregular, regular and sagittal forms of the triradiate system 

 can be distinguished, each of which may have an extra ray tacked 

 on, and so become quadriradiate. The fourth ray may be straight 

 or curved, long or short, smooth or spined, but all its variations 

 are quite independent of the variations of the rays of the basal 

 system. 



Although the spicules of Ascons often exhibit very definite and 

 symmetrical patterns, it is obvious that their forms do not in the least 

 resemble those of the inorganic calcite crystal, and from their outward 

 appearance it would be impossible even to suspect them to have any- 

 thin f^ in common with the calcite crystal. In fact, several features 

 seen in the spicules in question are the exact opposite of those charac- 

 teristic of crystals. Few things are so remarkable in crystals as the 

 fact that their parts are so connected together that one part cannot 

 vary independently of other parts, a property well seen in the laws 

 reo'ulatiug the addition of new faces during growth. But in the 

 spicule any part can vary independently of the rest. The rod -like 

 forms always have the two ends unlike ; the triradiate may have all 

 the rays unlike, and of different sizes ; and it is the rarest thing to 

 find a quadriradiate with the apical ray similar to the basal rays. 



In spite of their remarkable divergence from the usual crystalline 

 form, however, it is easy to prove not only that the spicules are 



