REPORT ON THE HEXACTINELLIDA. 



499 



Cera.tosa. 



MoJUKeotiia. 



Tetmetine^t/a, 



LitA/^fiWft. 



or predominantly, that form of skeletal element from which the rest have developed, we 

 must, after determining this fundamental form, strive to understand why in each 

 individual stem the specific form of spicule must be what it is. 



In regard to the calcareous sponges, Haeckel expresses, in his epoch-making monograph, 

 the conviction, based both ou anatomical and em- 

 bryological facts, that the fundamental and primi- 

 tive form of all calcareous sponge spicules is the 

 regular triradiate form, and in association with that, 

 perhaps, though less widely distributed, the simple 

 rod-like form. But by " regular triradiate " 

 Haeckel means a three-rayed spicule, in which the 

 rays, perfectly similar in size and form, meet at 

 equal angles of 120°, and all lie in one plane. 

 Haeckel ^ shows that even yet the triradiate spicules 

 are throughout the firm supports of the body, while 

 the tetracts occur as the protective weapons of the 

 gastral surface, or the rods of the dermal ; and he 

 draws the conclusion " that the triradiate forms 

 originally and primarily played the principal role, 

 while the four-rayed spicules first arose only as 

 internal adaptive modifications on the gastral sur- 

 face, and the rod-like forms as external and therefore 

 secondary diS"erentiations." 



As of special importance in regard to the forma- 

 tion of this specific form of spicule (triradiate) in 

 the soft parts of the primitive calcareous sponges, 

 Haeckel notes (p. 377) a peculiar process of 

 " bioerystallisation, i.e., a compromise of the 

 crystallising tendency of carbonate of lime and 

 the organic processes of the protoplasm." The 



calcareous spicules of the Calcispongias are, according to Haeckel, to be regarded as 

 " biocrystals or form -individuals occupying a median position between an inorganic crystal 

 and an organic secretion, and in their origin expressing a compromise between the 

 crystallisation of carbonate of lime and the formative activity of the fused cells of the 

 syncytium." " The original and fundamental form of all three-rayed and four-rayed 

 spicules is the absolutely regular triact, which may be considered as a hemiaxonial 

 form of the hexagonal crystal system, in which the carbonate of lime crystallises as 

 calc-spar." 



' Lot. cif., Bd. i. p. 352. 



CaCcaTva 



ITexaaUnellida . 



Fig. 9. — Genealogical Tree of the Porifera. 



