SPICULES OF SYNAPTA AND AURIOULARIA LARVA. 483 



Studies in Spicule Formation. 



VII. — The Scleroblastic Development of the Plate-and-Anchor 

 Spicules of Synapta, and of the Wheel Spicules of the 

 Auricularia Larva. 



By 

 \¥. Woodland, 



Tlie Zoological Laboratory, King's College, London. 



With Plates 29 and 30, and 6 Text-figures. 



Part I. — The Spicules of Synapta inhgerens and S. digitata. 



Introductory. 



The complete morphogenesis of the curious aggregate 

 (8) plate-and-anchor spicules of Synapta inha3rens has 

 been previously described and figured by Semon (6), but, 

 with the exception of my description of the disposition of the 

 scleroplasm associated with the adult spicules (7), no account 

 of the part played by the living tissues of the organism in the 

 production of these wonderful structures has hitherto been 

 published. 



These plate-and-anchor spicules are so well known that it 

 is not necessary for me to do more than briefly mention their 

 more obvious characteristics. Each spicule, as seen in the 

 integument of Synapta inheerens, e.g., consists of two 

 parts — the anchor and the plate — and these two parts are 

 quite separate from each other. The anchor consists of a 

 shaft, a bow (with its two arms), and a handle — to use the 

 current terminology ; the plate, on the other hand, has several 

 large perforations, and is somewhat more pointed in shape 

 where it comes into apposition with the handle of the anchor 

 than at the opposite side (fig. 23). This more pointed region 



