GEODIA A(;ASSIZII. 



135 



In the choanosome of several of the specimens of Geodia agassizii, particu- 

 larly the adult one from Station 3099, and the young one from Station 4228, 

 numerous young sterrasters in various stages of development have been observed. 

 In the adult specimens all these young sterrasters exhibit the well-known 

 form of spheres composed of very numerous, regularly distributed, slender, 

 radial rays. In the young specimen from Station 4228 young sterrasters of this 

 kind are also abundant, but here besides these ordinary forms numerous other 

 asters, similar in dimension, but very different in appearance occur. A close 

 examination showed the latter to be the young forms of the sterroids. By 

 comparing a large number of these and the ordinary young sterrasters with 

 each other and with the fully-developed ones I was able to trace the develop- 

 ment both of the normal sterrasters and the sterroids. 



On Plate 30, figs. 11-17, Plate 32, figs. 29-31, and Plate 33, figs. 1-8, 

 two series of developmental stages of the normal thin-rayed sterrasters, the first 

 from an adult specimen from Station 3188, the second from the young specimen 

 from Station 4228, are represented. One of the youngest stages observed, Plate 

 33, figs. 1, 2, is a sphere 17 /x in diameter. This young sterraster consists of 

 about 460 straight and exceedingly slender, thread-like, concentric rays, which 

 are equal in length, regularly distributed, and jointed proximally. Adjacent rays 

 enclose angles of about 10°. As the spicule grows these rays increase in thick- 

 ness, their proximal parts thickening first and this process of thickening then 

 extending distally. The basal thickening parts of the rays coalesce, as they 

 come in contact with each other, to form a solid centrum. In the next stage 

 (Plate 33, figs. 3, 4) the basal parts of the rays protruding from the solid cen- 

 trum thus formed show a slight bulbous thickening which increases so that in 

 young sterrasters 50 /x in diameter (Plate 33, figs. 5-7) the rays are at the 

 base already 2 /x thick, their distal ends, however, being still quite thin. In this 

 stage each ray appears as a cone, widened below like a bulb, and drawn out to 

 a fine thread above. As development progresses the thickening of the rays 

 extends farther and farther towards their distal ends (Plate 30, figs. 11-13), the 

 whole sterraster and its solid centrum increase in size, and the fine terminal 

 points of the rays become shorter and shorter, until they are entirely enveloped 

 in the ascending thickening and thus altogether disappear. In this stage (Plate 

 30, fig. 14; Plate 33, figs. 7, 8) the young normal sterrasters appear as solid, at 

 first still fairly spherical, central masses from which cylindrical rays, which 

 stand close together and are about 4 fi thick and simply rounded at the end, 

 protrude. Without changing much in appearance, these young sterrasters in- 



