132 Papers from the Department of Marine Biology. 
simpe rods united at short intervals by more or less irregular con- 
necting bars (fig. 2). From the nodes short spines project toward the 
periphery. Near the base of the arm these spine-like processes become 
longer and they themselves give out branches which unite with one 
another, so that an irregular lattice-work is formed continuous with 
that in the body of the pluteus. 
In the pluteus possessing 3 larval arms, the transverse rods are 
shorter and are met by a third rod to which those supporting this arm 
are attached. All of these bend downward at their ends, giving off 
long sweeping branches directed posteriorly. In the larva with 5 arms 
there is also a corresponding number of transverse rods. 
THE PERMANENT SKELETON. 
PLATES. 
Although the specimens are all of the same age, a considerable varia- 
tion exists among them as to degree of development of the permanent 
plates, so that I have been able to obtain a series showing more fully 
the early development of these and of the simultaneously developing 
spines than of any of the other structures (figs. 4-16). 
In those specimens showing the most advanced stage of development, 
the permanent skeleton consists of a circle of 5 plates surrounding the 
region where the mouth is to form, one of which lies anteriorly, 2 lat- 
erally, and 2 posteriorly (fig. 16). Outside of these is a circle of 10; 
2 lie back of the anterior, 1 back of each lateral, and 3 are clustered 
behind each of the 2 posterior plates. As a consequence the mouth 
anlage has an eccentric position, somewhat to the anterior of the result- 
ing skeletal structure. The plates are all similar and are those to 
which the first spines are attached. Many of the specimens are in this 
stage and possess 15 spines and plates. But new ones are constantly 
developing which crowd the surrounding area as well as push in among 
those of the first two rows, so that later no definite arrangement can 
be determined. 
Each plate commences as a triradiate spicule (fig. 4), the radii lying 
in the same plane and at angles of 120° to one another. Afterwards a 
fourth branch arises from the center and pushes outward, and it is to 
this that the spine later becomes attached. Figures 4 to 7 and 10 to 
14 give a series of these developing plates. The 3 primary radii soon 
divide, each time at about the same angle (figs. 5, 6, 10), so that by 
repeated division 3 hexagonal spaces are inclosed (figs. 10,14). By the 
continued pushing out and rebranching of the radii a plate like that 
shown in figure 15 is formed. 
SPINES. 
Early in the development of the structure described above, a second 
spicule, with 6 rays instead of 3, may be observed lying near each plate 
and external to it (figs. 5 and 6). Each ray of this new structure soon 
