EUPLECTELLA. 55 



like diactin is derived from a simple hexactin by the suppres- 

 sion of the four crueiately disposed ra3^s. 



Of special interest is ^. suberea W. Thoms. in that it gives 

 some clue to the genetic relation between the oxyhexaster and 

 a form of discohexasters, the onychaster (PL X, figs. 12, 20, 21), 

 which appears strikingly like the former but is distinguished by 

 having exceedingly fine claw-like or branch-like appendages at the 

 outer end of the terminals. It is known with certainty that certain 

 individuals of that species possess true oxyhexasters in abundance 

 (Schulze '99, p. 19 ; Topsent '92, p. 24). Now, Topsent {I.e.) 

 made the interesting observation that in a specimen examined by 

 him the oxyhexasters were entirely replaced by onychasters, and 

 further that in another specimen there occurred neither the one 

 nor the other in typical development but a form combining the 

 characters of both in that one or more of the terminals bore 

 each a single hook at the free end, while the rest terminated in 

 simple points. The onychaster was evidently also seen by 

 W. Thompson in one or the other of the specimens obtained by 

 the ' Challenger,' for we see one represented in fig. 8, PI. V, of 

 the Challenger Report, which plate was prepared by that eminent 

 naturalist, although in the text of the report Schulze held the 

 spicule of that figure to be of extrinsic origin. With respect to 

 what might be called the onycho-oxyhexaster discovered by 

 Topsent, that writer justly concludes that it represents a form 

 intermediate between discohexasters and oxyhexasters. Cases 

 suggestive of the same transition are also known in the genus 

 Aphrocallistes [A. ramosus, hocagei ; Schulze '95, pp. 77, 80). 



Which of the two hexaster-forms, the onychaster or the 

 oxyhexaster, is then the more primitive ? Taken alone, the onycho- 

 oxyhexaster would look just as much like an oxyhexaster on the 



