14 INVERTEBRATE PALAEONTOLOGY 



of the animals that built them ; but those that are 

 made up of loose scales or ossicles (e.g. many Sponges, 

 Holothurians and Arthropods), held in place by muscles 

 or ligaments, will collapse into fragments on the death 

 of their owners, and reconstruction will be a matter of 

 difficulty and uncertainty. 



The few animals that build skeletal structures of 

 silica (chiefly Radiolaria and Sponges) secrete that 

 mineral in the opaline or amorphous condition its least 

 durable form liable to removal by solution in alkaline 

 water. Calcareous shells or skeletons are sometimes 

 made of calcite, a form of calcium carbonate which is 

 relatively stable (see PI. iii. fig. 4), but none the less 

 readily soluble in acidulated water ; those that are com- 

 posed of the rhombic Aragonite are particularly prone 

 to solution and decay (see PI. iii. fig. 5). Chitin, which 

 is an organic substance far more stable than ordinary 

 soft tissues (comparable in this respect with the woody 

 fibre of plants), can be preserved only if hermetically 

 sealed up soon after death (see PL iv. fig. i); even then 

 it often passes into a carbonized and partly collapsed 

 condition. Even when mixed with various salts of 

 lime (as in many large Crustacea), it is still transient 

 if exposed to atmospheric action. Hence the possession 

 of shelly structures in no way ensures the fossilization 

 of an organism, although it is necessary if such a con- 

 summation is to be normally attainable. 



Again, secretion of mineral matter is the especial 

 attribute of animals in adult stages ; it is rarely or but 

 feebly developed in young or larval forms. Hence a 

 large proportion of the material available for palaeonto- 

 logical study consists of fully grown specimens ; early 

 developmental stages have rarely left recognizable 

 traces. Since the history of an individual life seems 

 to present an epitome of the evolution of the race to 



