ERYLUS SOLLASII. 



285 



Young aspidasters were found in considerable numbers scattered through- 

 out the choanosome in several specimens. They are imbedded in the ground 

 substance. A special membrane or plasmatic sheath enclosing them could not 

 be made out even in sections strongly stained with aniline-blue. 



The youngest (smallest) aspidasters observed (Plate 2, fig. 14) were oval 

 discs, about 25 n long and 10 /f broad, and composed of numerous exceedingly 

 slender and perfectly straight rays which radiated from a granular centrum 

 abcjut 3 /( in diameter. These rays are, at first, quite isolated. They grow in 

 length and in thickness and so the whole aspidaster increases in size (Plate 1, 

 fig. 3Gd; Plate 3, fig. 25d) and the basal parts of the rays become united. The 

 solid, central mass of silica thus produced forms a disc from the margin of which 

 the still isolated, distal parts of the rays protrude (Plate 2, fig. 19; Plate 3, 

 fig. 16). When this stage is reached the longitudinal growth of the rays slows 

 down or ceases altogether, while the transverse growth of the rays, that is their 

 increase in thickness, continues. In consequence the marginal spines become 

 joined more and more (Plate 2, figs. 20, 21) until they entirely lose their individ- 

 uality, the margin of the aspidaster becoming quite smooth. In young forms 

 of regular aspidasters this smooth, non-serrated margin is continuous (Plate 3, 

 fig. 11), in young forms of irregular ones lobose (Plate 2, fig. 15). Not only the 

 margin Ijut also the two broad faces of such young aspidasters are smooth. 

 Their smoothness in this stage constitutes the chief difference between them 

 and the sterrasters of Geodia, which do not pass through a smooth stage during 

 their development. Later small, spine-like protuberances make their appear- 

 ance on the surface of the smooth young aspidaster. These develop into the 

 protruding rays with terminal verticils of lateral spines, which have been 

 described above. 



LOCALITIES .AND N.ATURE OF ENVIRONMENT. 



