ERYLUS ROTTJNDUS. 



299 



The aster-like rhabd-clnsters (Plate 7, figs. 4-10), which have been found 

 only in var. cidaris, are, in my opinion, to be considered as derivates of ordinary 

 rhabds. They appear as smooth oxyasters, composed of from about fifteen to 

 thirty concentric, straight rays fairly uniform in thickness, but differing exceed- 

 ingly in length, and distributed very irregularly. From four to ten of the rays 

 appear properly developed; these are conical, more rapidly attenuated distally 

 than proximally, and pointed. The other rays are rudimentary, very short, 

 cylindrical, and terminally rounded. These rudimentary rays together form a 

 kind of lobose centrum, from which the longer, pointed rays arise. The rhal)d- 

 clusters are 125-180 /i in total diameter. Their rays are 5-8 /< thick and the 

 longest one of the whole cluster is 70-100 // long. 



The branched amphioxcs (Plate 7, figs. 1-3) also occurring in this variety, 

 which I consider as transitions between the clusters and the ordinary rhabds, 

 are 410-520 /« long and 9-13 /i thick. They bear from one to four straight 

 branch-rays. 



The 7nicrorhabds (Plate 5, fig. 27e; Plate 6, figs. 30-35; Plate 7, figs. 46-51, 

 53a, 54a, 55, 56a, 57a, 60a, 61 73, 75a, 79) are for the most part simple, isoactine, 

 gradually or rather abruptly pointed or, more rarely blunt, uniformly curved, 

 and usually slightly centrotyle amphioxes. The tyle, never large, is often so 

 insignificant as to be hardly visible, and many of these spicules seem to have no 

 central thickening at all. In the forms A, B, and C of var. typica (Plate 6, figs. 

 30-32; Plate 7, figs. 46, 48 51, 53a, 54a, 55, 56a) nearly all the microrhabds have 

 a distinct central tyle. In the microrhabds of var. cidaris (Plate 7, figs. 75a, 79) 

 the tyle is not so well developed, and in most of the microrhabds of var. jnega- 

 rhabda (Plate 6, figs. 33-35, Plate 7, figs. 57a, 66-73) and var. typica, form D 

 (Plate 7, fig. 47), the tyle is hardly perceptible or absent altogether. Besides 

 these regular, simple, and isoactine microrhabds a few anisoactine ones, with one 

 actine reduced in length and rounded at the end, and a few with small branch- 

 rays near one end, have been observed in var. cidaris, in both forms of var. 

 megarhabda, and in form C of var. typica. The microrhabds are 30-98 /( long and 

 1.5-7.5 fi thick. Those of var. megarhabda are considerably larger (maximum 

 averages of three 82.3-95.7 by 5.3-6.7 /«) than those of the other two varieties 

 (maximum averages of three 54-61.7 by 3-3.8 and 66.1 by 4.2 // respectively). 



