548 B. A. M INCH IN. 



Dieyer, in his studies on skeletal formations (1892), is more 

 ambitious than either Haeckel or Schulze, since he tries to 

 find a theory to explain the forms of the spicules, not only in 

 sponges, but Radiolaria and Echinoderms as well. Like 

 Sollas, he invokes mechanical principles, but only to explain a 

 supposed universal type of primitive sclerite. He will have 

 nothing of either crystallisation or adaptation in explanation of 

 the forms of spicules. Crystallisation he rejects, with Schulze 

 and Sollas, as an agency which determines the forms, on the 

 ground of the occurrence of spicules similar in form but com- 

 posed of very different materials, such as lime or silica, or even 

 substances of organic nature. Hence he concludes that the 

 morphology of the sponge spicule is quite independent of the 

 nature of the materials composing it (p. 299). Adaptation he 

 considers inadequate, on the ground that natural selection is 

 *' an externally regulative," not an 'Mnternally formative" 

 principle (p. 349). 



Dreyer regards the tetraxon type of spicule as the primitive 

 form of skeletal element, not only for sponges, but also for 

 E-hizopods and Echinoderms. To explain this universally re- 

 curring type of spicule it is necessary to seek for some constant 

 cause; and this he finds in the vesicular structure of 

 living matter. All living bodies are built up of three orders 

 of vesicular elements, namely, (1) cells, (2) vacuoles, and (3) 

 the alveoli of the protoplasmic framework, according to 

 Biitschli's theory of the ultimate structure of protoplasm, with 

 which he agrees (p. 350). The spicules being formed by living 

 bodies, and being therefore deposited in the midst of these 

 vesicular structures, have primitively the tetraxon form as a 

 direct mechanical result of vesicular tension (Blasenspannung). 

 Each such primitive tetraxon is, in fact, regarded by the author 

 as laid down in the nodes of the alveolar framework in such a 

 way that each arm lies in the interspace between three of the 

 four contiguous alveoli, which naturally touch each other at a 

 node. 



The most unfortunate gap which at once presents itself in 

 this theory is the fact of its being totally inapplicable to the 



