544 E. A. MINCHIN. 



have been instrumental in pioducing the fundamental types 

 under which they occur. 



Four theories have been put forward to explain the forms of 

 the spicules of Calcarea or of sponges generally, which we may 

 term respectively the Biocrystallisation Theory of Haeckel, the 

 Adaptation Theory of Schulze, the Mechanical Theory of 

 Sollas, and the Alveolar Theory of Dreyer. 



Haeckel, whose views on the secretion of spicules we have 

 already noticed, regarded calcareous spicules as the products of 

 a process of '^ biocrystallisation," "i.e. a combination of the 

 crystallising activity of calcium carbonate and the organis- 

 ing activity of the sarcodine.^' Calcareous spicules are 

 " biocrystals, i. e. form individuals, which represent a mean 

 between an inorganic crystal and an organic secretion." 

 " Their first origin depends on a compromise between the 

 crystallising efforts of the calcium carbonate and the forma- 

 tive activity of the fused cells of the syncytium." The 

 primitive secretion of calcium carbonate assumed a semicrys- 

 talline nature, and gave rise to bodies 'Svhich were utilised 

 by natural selection as spicules for building up a skeleton, 

 and which afterwards, by the interaction of adaptation and 

 heredity, became modified in form and difi'erentiated in very 

 various ways in the struggle for existence" (1872, p. 377).^ 



Haeckel considered the primary forms of spicule in the 

 Calcarea to be the triradiates, and the monaxou or acerate forms. 

 He regarded the quadriradiate form as undoubtedly secondary, 

 and derived from the triradiate by addition of the gastral ray. 

 He discussed the relationship between the two primary forms, 

 and considered (p. 350) the three possible theories — (1) that the 

 monaxon and triradiate spicules are independent formations ; 



(2) that the monaxon spicule has arisen from the triradiate; 



(3) that the triradiate spicule has arisen from the monaxons. 



1 Schmidt at an earlier date (1870, p. 4) had attempted to explain the 

 regular tetraxon siliceous spicule by the action of crystallisation, but con- 

 sidered any such explanation inapplicable to calcareous spicules. Schmidt 

 evidently thought that in siliceous sponge spicules the silica was in the 

 crystalline form of quartz, whereas it is now known to be in the colloid form 

 of opal. 



