1895.] the Triradiate Spicules of Leucosolenia. 205 



by each cell. In many instances it appears as if the three rays were 

 formed quite separately and afterwards fused at the centre. 



The three outer cells soon lose their rounded form, and by throwing 

 out processes, assume an amoeboid appearance. After the spicule 

 rays have attained a length of 10 or 15 /t, the three outer cells are no 

 more to be found, having apparently rejoined the flat epithelium from 

 whence they came. The three inner cells alone secrete the rays and 

 continue to do so until the spicule is full grown. 



The spicule rays soon appear to project beyond their formative 

 cells, but are in reality covered by a thin layer of protoplasm. At 

 the same time, the spicule sheath makes its appearance as a 

 denser layer of substance between the protoplasm of the formative 

 cell and the calcareous spicule, and it is by continued calcification of 

 the sheath that the spicule grows. 



The spicule rays attain their full thickness at their bases before 

 they have reached their full length. The formative cells remain at 

 the bases of the rays until this portion is built up to its full thick- 

 ness. Each formative cell then migrates along its ray towards the 

 tip, building up the ray to its full thickness as it goes. In the 

 fully formed ray the formative cell is found adherent to the extreme 

 tip. 



Theoretical considerations. 



(1.) The origin of the spicule- form ing cells that is to say, of the 

 whole connective tissue system in these sponges from the external 

 flat epithelium, is another nail in the coffin of the so-called mesoderm 

 in these forms. Sponges are to be regarded as two-layered animals, 

 composed of a dermal and a gastral layer. The dermal layer is 

 differentiated into (1) an external flat contractile epithelium, the 

 neuro-muscular system, and (2) an internal connective tissue layer. 

 The gastral layer consists of the collar cells. The amoeboid wander- 

 ing cells are perhaps also to be reckoned with the gastral layer. 



(2.) The fact that each ray of a triradiate spicule is formed by a 

 single cell, shows that each triradiate spicule must be regarded as 

 derived from the fusion of three originally separate monactinal 

 spicnles. This supports Schulze's theory, namely, that the triradiates 

 of the more primitive Ascons have arisen as an adaptation to the 

 structure of the spouge, and goes against Dreyer's theory that the 

 primitive spicule of all sponges is a tetraxon, a form explained by 

 him as the direct mechanical outcome of the vesicular structure of 

 living bodies. 



